[David Cooper (OP)]

Can 40 x 2500mAh AA batteries (cost £100) replace one 100Ah lead acid battery (cost £60)? I'm not suggesting this for use in a car, but for small boats (specifically sailing dinghies) where weight is important. There are electric outboard motors available (sometimes called trolling motors - popular with anglers when noise isn't wanted), but they're usually powered by heavy lead batteries. I would like to be able to use such a motor with a much lighter power source, mainly so that it becomes practical to carry 4 or 5 hundred Ah of power without the weight becoming prohibitive.

Lithium batteries of the kind used in electric cars might be best in terms of grunt per kg, but the problem with them is that you practically have to keep them on life support all the time - they don't like being left flat for long and they don't like being kept full charged either. NiMH looks as if it could be a better alternative, just so long as you always run batteries close to flat before recharging them fully.

My idea would be to use several blocks of 40 AA batteries, putting 8 sets of 5 batteries in series (to add up to 12 volts), with each set of 5 connected in parallel. (By using them in parallel, it should stop them getting hot and lengthen their lifespan.) I know that there can be problems when using multiple batteries if some of them run down before others (they can be reverse charged, and that damages them) but it would be possible to identify the individual batteries that run down most quickly or slowly and move them into other sets more like themselves so that they are better matched.

So, has anyone ever tried doing anything of this kind? I don't want to repeat an expensive experiment that may be doomed to failure from the start if it turns out that combining too many of these batteries together results in too high a battery death rate. Even if running them down in such a block isn't a problem, it may be that trying to charge them as a block will be the killer instead. The purpose behind all this is to make long coastal and river journeys safer and less gruelling, but without needing to carry around a tank of explosive liquid and a noisy oil-spewing machine attached to the transom. It should also be possible to do some of the charging with a solar panel, and ultimately it may be possible to use a sail which serves as a solar panel.

[CliffordK]

I think you're off by about a factor of 10.

40 NiMH batteries (2.5 AH, 1.2V) arranged in parallel is equivalent to 1 battery, 1.2V, 100AH.   

Or, you can arrange 10 banks of 4 in series/parallel to make a 12V, 10AH battery.

So, to make them equivalent to your 100AH, 12V deep cycle car battery, you'll need 10 of these blocks of batteries, or 400 of your AA batteries in total.

Going with D cells, you would need about 1/4 that number, or 100 batteries or so, which at least will make it easier to wire up.  Perhaps even consider larger batteries such as F size.

I believe the original Teslas used blocks of small Lithium batteries, although I think they also had fairly complex electronic control circuitry.  But, the Lithium batteries may be more touchy with over charging as well as low voltage cycling than the NiMH batteries. 

You may also be able to find used Prius battery cells, 7.2V, 6.5AH.  Your trolling motor should run just fine wired to 14.4V.

I'd certainly go for some kind of a smart charger.  Trickle Charging might be ok, but most of the 14.4V NiMH chargers I'm seeing are for less than 5AH batteries, and may struggle with a 100+AH battery pack.  This one should work with your pack, if you go with 12V. http://www.absopulse.com/Absopulse_PressRel_BCP_130-PEL.php [Links inactive - To make links active and clickable, login or click here to register], but I'm not seeing retail pricing on it.

I'm not sure what it would take to convert a standard auto charger for use with NiMH batteries.

Personally I never just "run down" batteries, although it may not be bad to wait to recharge until they are mostly depleted, assuming you can wait that long with ordinary use.  Lead Acid batteries should probably be stored full though.

Oh, one note about running 12V car batteries in parallel.  Kill one cell, and it will quickly kill the whole set.  I don't know if NiMH batteries suffer with the same low cell problems that lead acid batteries have.

[RD]

The maximum current drain possible may be different for the two types of batteries.
Electric motors can draw high currents , you should check the specifications of the non-lead-acid batteries to see if are capable of constantly delivering that high current.

Also find out what impact high current drain has on the lifespan of the non-lead-acid batteries,
to see if they make economic sense as a replacement.

[David Cooper (OP)]

I think you're off by about a factor of 10.

40 NiMH batteries (2.5 AH, 1.2V) arranged in parallel is equivalent to 1 battery, 1.2V, 100AH.

That makes more sense - I was hoping you might not need to equalise the voltages for the Ah figure and that it was a measure of the total amount of energy stored, but the small size of the resulting pack did seem too good to be true. Thanks for the injection of realism.

So, it's probably 400 batteries needed, at a minimum cost of £500 (if cheaper brands can be trusted) or more likely £1000. Having now got round to weighing 4 x 2500mAh NiMH AA batteries, I find that that comes out at a fraction under 100g, so 400 would be 10kg. That's still pretty good, because a lead acid 12v 100Ah battery weighs about 30kg, and a lithium one weighs 15kg, although the stats on a few suggest they could come down to 11kg (eg. http://www.alibaba.com/product-detail/li-ion-battery-pack-12v-100ah_1372540256.html [Links inactive - To make links active and clickable, login or click here to register] - many of the batteries on offer have contradictory stats and appear to contain errors, so 270g probably means 2700g, and four lots of that would be needed for 12v).

Going with D cells, you would need about 1/4 that number, or 100 batteries or so, which at least will make it easier to wire up.  Perhaps even consider larger batteries such as F size.

The best D cells I've found are 3000mAh, so there's not much gain there and an enormous amount of extra bulk. The F ones aren't great either (never heard of them before): http://cellpacksolutions.co.uk/online-shop/yuasa-1fm130-f-rechargeable-battery/ [Links inactive - To make links active and clickable, login or click here to register] is 13,000mAh, so that's worth 5 AA batteries, but it's actually double the weight for the power, and more expensive.

http://www.absopulse.com/Absopulse_PressRel_BCP_130-PEL.php [Links inactive - To make links active and clickable, login or click here to register]

So it has been done, but it looks as if NiMH isn't really in the game all the same - all the 12v 100Ah batteries I've found are either lead acid or lithium. I didn't find the lithium ones at Amazon as they don't ordinarily appear to be sold directly to the public, so it's only today that I've found them on Alibaba. The advantages of lithium must be sufficient to have pushed NiMH out of the market, probably because it's too inconvenient to run batteries right down and charge them fully every time in most applications; something that's clearly less practical if you're only using one battery. It also looks as if they can handle twice as many charges as NiMH, so that may be the crunch. I should probably just go for li-Ion and aim to use them heavily to get best value out of them.

Oh, one note about running 12V car batteries in parallel.  Kill one cell, and it will quickly kill the whole set.  I don't know if NiMH batteries suffer with the same low cell problems that lead acid batteries have.

It's probably best not to cobble something together then - I'll look for something that's already on the market in a form that doesn't need modification. Four 12v 100Ah batteries plus two heavy chargers is probably what I'll need to go for, given that the charge times are typically six hours and there'll only be time to charge two each night with a single charger. In terms of weight, that'll be like carrying an extra person in the boat, but that's acceptable.

[homebrewer]

Hi David,
 
Before you undertake any electrical work, try to take a close look at the ballast arrangements in your craft, (Can some ballast be replaced by lead acid batteries ?) and consider the fixed installation of a solar panel for charging purposes.

In as far as the choice of batteries goes, I would go for lead acid batteries in any case, but please stay clear of Lithium batteries. As Lithium batteries and water are two incompatible friends and have a potential to cause severe fires in your craft.

Good luck.

[CliffordK]

There are some large format NiMH batteries.  As mentioned, the Prius batteries are NiMH, I think, as well as the batteries in some of the early electric vehicles.

However, there was a patent on the NiMH batteries which has very much damaged the production of large scale NiMH batteries.  Most of the patents are due to expire sometime this year (2014).  It is hard to say if there will be a resurgence of the technology once the patent encumbrance is lifted.  To a large extent the batteries have already been supplanted by various Lithium technologies.  However, as you mentioned, the NiMH batteries may be more robust than Lithium technologies for use in many applications.  I would expect Boeing would evaluate using NiMH batteries rather than Lithium on their Dreamliner.

As far as using Amps as a rating on batteries.  As you know, Watts=Amps X Volts. 

The Amp rating is handy because it is independent of volts.  Put ten, 2.5AH, 1.2V batteries in series, and one still has 2.5AH, but jumps to 12V. 

Likewise, the discharge current is rated in either Amps, or percent of "capacity" in AH, so a 100AH battery (independent of voltage) might be rated for a 0.3C discharge, or 30 Amp continuous discharge.

How is longevity rated on your little AA batteries? 

Another option to consider is battery packs already designed for various hand tools.  They should be already setup with necessary chargers, monitoring equipment, and etc.

[alancalverd]

Beware of assembling large stacks of cells that aren't designed for such assembly! You can get away with 10 x AA NiMH cells in series because in the event of one failing open-circuit the voltage across it will only be 10.8V. But if you put 40 cells in parallel and one fails short-circuit, the prospective fault current through the dud cell is several hundred amps and more than likely to burn a hole in your boat.

Face it, lead-acid batteries are crude, oldfashioned, heavy, but extremely robust, tolerant and reliable for high current demand. There's a good reason why they are the only approved technology for most small planes, and judging from the problems Boeing are having with Dreamliner batteries, I'd rather have a PbAc in a boat than anything else right now. 

[David Cooper (OP)]

Hi homebrewer,

Hi David,
 
Before you undertake any electrical work, try to take a close look at the ballast arrangements in your craft, (Can some ballast be replaced by lead acid batteries ?) and consider the fixed installation of a solar panel for charging purposes.

There is no ballast other than crew weight. There are two different kinds of sailing dinghy I'm looking at using this in (one of which is a multihull which only exists on the drawing board at the moment). The monohull has buoyancy tanks within which the batteries could be contained such that water cannot get to them unless there is a catastrophic failure of the hull. They would also be in waterproof containers within that, so the only real danger to them is a short circuit at some point along the wires connecting them to the motor (and to other devices). Clearly they could go on fire at any time for no obvious reason, even though it's rare for this to happen, so a means to detect that and eject them quickly would need to be devised just in case. [The modifications to the hull will break the rules for racing, but that isn't important if a boat's being adapted specifically for travel.]

In as far as the choice of batteries goes, I would go for lead acid batteries in any case, but please stay clear of Lithium batteries. As Lithium batteries and water are two incompatible friends and have a potential to cause severe fires in your craft.

Well, lead acid is still an attractive option because of the low cost, despite the extra weight, but it would constrain the amount that can be carried. I have missed a trick though, because I realise now that I may be able to make do with a lot less power than I had originally imagined. If you're trying to get up a river where in many places the current is just too fast to row against, using a motor in addition to rowing could make a journey possible that wouldn't otherwise be, but if you run out of battery power you could get stuck somewhere awkward (and in a remote location where finding a place to recharge the batteries is not likely to happen). Carrying more power makes that less likely to happen, but just by anchoring in the current it should be possible to use a turbine to generate power to charge the battery back up. It may even be possible to use a supercapacitor to store a lot of power quickly and then release it quickly too to get up much faster-flowing sections of river. This is something that needs to be experimented with, so the thing to do is start by using cheaper components to test what's actually possible before buying any of the expensive stuff that might be used later on. I should therefore start out with one lead acid battery and find out exactly what its limits are.

What led me to think about all this in the first place was a book called The Unlikely Voyage of Jack de Crow in which a Mirror dinghy (of that name) is sailed and rowed via rivers and canals (plus a Channel crossing) from Wales to the Black Sea. I know a number of people who are interested in the idea of attempting that or similar journeys, but it really is far to gruelling to rely on rowing whenever sailing isn't possible. Worse than the currents in rivers are the strong tides in estuaries, but being able to run a motor for an hour in addition to rowing should be sufficient to make it safe in most cases. More power = greater safety, but the usual way to get it is to carry tanks of explosive liquids and to spew pollution everywhere while waking the dead with an ear-splitting racket. There is no perfect solution - carrying power is dangerous, but not having it is dangerous too, so it's a matter of finding the least lethal setup.

[David Cooper (OP)]

There are some large format NiMH batteries.  As mentioned, the Prius batteries are NiMH, I think, as well as the batteries in some of the early electric vehicles.

I missed that point about the Prius before - I thought it was lithium.

However, there was a patent on the NiMH batteries which has very much damaged the production of large scale NiMH batteries.  Most of the patents are due to expire sometime this year (2014).  It is hard to say if there will be a resurgence of the technology once the patent encumbrance is lifted.

That could change the whole game then, given that NiMH appears to be lighter for the power it holds, but time will tell, and it's probably best not to be a pioneer unless you're a manufacturer.

As far as using Amps as a rating on batteries.  As you know, Watts=Amps X Volts.

You're being to kind - as I may have known once, but had forgotten.

Likewise, the discharge current is rated in either Amps, or percent of "capacity" in AH, so a 100AH battery (independent of voltage) might be rated for a 0.3C discharge, or 30 Amp continuous discharge.

Thanks - that's cleared up another question for me.

How is longevity rated on your little AA batteries?

They usually say they're good for 1000 charges, but in reality they probably only manage half that with the capacity already falling off considerably. Fast chargers kill them much more quickly, so I normally use slow chargers which take about 24 hours. 12 hours is probably fine though.

Another option to consider is battery packs already designed for various hand tools.  They should be already setup with necessary chargers, monitoring equipment, and etc.

Yes - electric bicycle batteries have the right characteristics, but I know now where some of them get their batteries from and it would clearly be a lot better to buy them direct from the Chinese manufacturers who already offer to package them in the right way for whatever application you have in mind, so long as you're buying enough of them. Once the optimum power system for sailing dinghies is found, this could eventually become a business, although it isn't one that I'm interested in making a profit from - I just want the ideal product to be available because it will enable adventures, and many people could gain from it.

[CliffordK]

I wouldn't consider a capacitor as giving a boost for more than a few seconds.  And, really any "boost" would be due to an over-voltage which you might be able to wire directly with your batteries, assuming the batteries are capable of putting out the amps you require.  Of course the over-voltage does put your motor at somewhat of a risk.  I suppose the super cap may also help a bit with peak current draw, if you normally use far below what your batteries can put out, but occasionally hit higher current loads (many batteries have peak, and continuous current ratings).  Can you lug your boat motor?

Capacitors may be handy to remove ripple from whatever variable speed device you're using for better power utilization, although at peak power, you should be driving directly from the batteries.

You're talking about doing rowing, sailing, and whatever.  Everything depends on the size of your boat.  I've driven an Electric Ford Ranger using lead acid batteries.  They are HEAVY for pretty marginal range.  I'm considering an electric pedal assist vehicle, but will choose a design that is much lighter. 

Likewise, a boat is somewhat more forgiving with a bit of extra weight, but the weight is not entirely free.  Add 1000 lbs of batteries, and it will be a pain to row by hand upstream. 

Here is a list of http://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials [Links inactive - To make links active and clickable, login or click here to register].  The attraction of gasoline is 50 or so times the energy density of the Lithium Ion batteries. 

Anyway, there is an advantage of light, high energy density batteries, even if you would benefit from some ballast.  But, other than the advantage of being able to easily shift a water ballast, there isn't a lot of sense carrying around a boat load of water either.

[David Cooper (OP)]

Beware of assembling large stacks of cells that aren't designed for such assembly! You can get away with 10 x AA NiMH cells in series because in the event of one failing open-circuit the voltage across it will only be 10.8V. But if you put 40 cells in parallel and one fails short-circuit, the prospective fault current through the dud cell is several hundred amps and more than likely to burn a hole in your boat.

The same must apply to large lithium batteries because most of them appear to be made of many smaller cells, many of which must be wired up in parallel. Maybe that's why they can fail so catastrophically. The best thing to do is buy something that's tried and tested and which has a low failure rate, then to plan for how to handle a failure. Given that a phone battery can explode and kill the user, I should probably think very carefully about the location of the batteries. Best not to sit on them.

Face it, lead-acid batteries are crude, oldfashioned, heavy, but extremely robust, tolerant and reliable for high current demand. There's a good reason why they are the only approved technology for most small planes, and judging from the problems Boeing are having with Dreamliner batteries, I'd rather have a PbAc in a boat than anything else right now. 

I'm definitely warming to the idea of using lead acid and just having less power available.

[David Cooper (OP)]

I wouldn't consider a capacitor as giving a boost for more than a few seconds.  And, really any "boost" would be due to an over-voltage which you might be able to wire directly with your batteries, assuming the batteries are capable of putting out the amps you require.  Of course the over-voltage does put your motor at somewhat of a risk.  I suppose the super cap may also help a bit with peak current draw, if you normally use far below what your batteries can put out, but occasionally hit higher current loads (many batteries have peak, and continuous current ratings).  Can you lug your boat motor?

The supercapacitor idea is one that I haven't considered in any depth, but it just struck me that it could be used to store a lot of energy in a fairly short time while anchored in a current and using a turbine. That energy could then be put out much more slowly, but through a stronger motor than the battery can take full advantage of. They're being used in electric cars for storing energy from braking, and it then has to be released carefully so as not to destroy the tyres. The way I'm thinking of using them may not be a goer, but it's worth looking into just in case it is viable.

You're talking about doing rowing, sailing, and whatever.  Everything depends on the size of your boat.

The Mirror dinghy is 11ft long and lighter than a typical rowing boat. 100Ah may be sufficient, but 200 certainly ought to be. The other dinghy I want to use a power system on is a 20ft multihull made of carbon fibre - it could manage with the same power, but it can carry a lot more weight without affecting its sailing performance significantly, so there's no reason not to have 400Ah or more on it.

I've driven an Electric Ford Ranger using lead acid batteries.  They are HEAVY for pretty marginal range.  I'm considering an electric pedal assist vehicle, but will choose a design that is much lighter.

A pedal powered propeller is more efficient than rowing and it may be so even with electric transmission, so one of the things I want to explore is pedalling to power a dynamo and putting the power out through the electric outboard motor. There is also solar power to consider, so the trick is to work out how to combine all of that into a single package.

Likewise, a boat is somewhat more forgiving with a bit of extra weight, but the weight is not entirely free.  Add 1000 lbs of batteries, and it will be a pain to row by hand upstream.

That weight would be excessive, but if you're drawing power from it all it should still beat rowing. The thing is though, sailing a heavy boat isn't fun - sailing dinghies are fun to sail because they are light and responsive, so the last thing you want to do is kill that with weight. One 100Ah lead acid battery shouldn't spoil the party though, but there's other luggage that needs to be carried and it all adds up. Any weight saving is worth looking at.

Here is a list of http://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials [Links inactive - To make links active and clickable, login or click here to register].  The attraction of gasoline is 50 or so times the energy density of the Lithium Ion batteries.

The simplest solution is just to use a tank of petrol, but the racket kills the fun, which is why most people don't want an outboard motor on their sailing dinghy. They're also unreliable, failing to start up when you most need them. Hydrogen looks as if it has possibilities - a turbine could be used while anchored in currents or while sailing in strong winds to generate hydrogen which could then be put through a fuel cell whenever power is wanted. Storing it would be a problem though. I see that the list shows Li-Ion as much better than NiMH for energy density, but that doesn't appear to be reflected in the weights of batteries.

Anyway, there is an advantage of light, high energy density batteries, even if you would benefit from some ballast.  But, other than the advantage of being able to easily shift a water ballast, there isn't a lot of sense carrying around a boat load of water either.

With dinghies, the only good ballast is crew weight, or anything that can be moved with the crew. On the 20ft multihull which only exists on the drawing board at the moment, there's a microcabin that slides on rails and hooks up to a trapeze when out to the side of the boat (to keep the weight of the rails down), so any weight inside that cabin is useful ballast. That is where the batteries would be located, but they need to be there for another reason too, because the idea is that this cabin can be detached from the boat and converted into a wheeled electric vehicle. This boat design is aimed at fast coastal racing and travel (hydrofoiling in winds over 8 knots and with boatspeeds of 30 knots plus in ideal conditions), while giving you land transport and a capsule-hotel level of accommodation whenever you're in port. It should also be capable of safe sea crossings on extreme routes such as Shetland to Norway or Iceland, being fast enough to outrun storms. Lithium batteries are the most desirable power solution for this boat, but building the thing's a long way off in the future. [It won't go up on hydrofoils under electric power - it will only go at rowing speed in that mode.] The priority is to test electric power systems in a small monohull first while concentrating on designing the multihull to be fast as a sailing dinghy and only adding the electric power to it later on. Having and maintaining sailing performance is the most important thing, because that's what makes using these boats for extreme travel such fun, but some kind of alternative means of powering them is needed for times when there is no wind and when currents make things impossible or dangerous. Electric power is also going to be needed for recharging batteries used in other pieces of equipment (such as a computer and cameras) which is another reason why basing it on battery power is preferred over petrol.

[syhprum]

I visited a small museum at Windermere that had a collection of electrically powered boats that were very popular in Edwardian times although that said it was rare to find any surviving craft because of the destructive effect of leaking acid.
This is something to consider if using Lead acid batteries.
 

[David Cooper (OP)]

It should be possible to contain that, but it does highlight how bad the situation is when a poisonous element working with a vicious acid is still one of the main contenders. At least cadmium is out of the game now though. Realistically there can be no safe solution, because energy itself is dangerous and packing more of it into the same space makes it more so, unless it's in a form that doesn't let you use it easily (i.e. without a nuclear reactor). I suppose one of the purest ways to store energy would be a room-temperature superconducting flywheel in a vacuum, but even there it's easy to imagine that it could get loose and kill lots of people. A dangerous and inconvenient solution is inevitable. I suspect it will end up being lithium batteries or hydrogen.

[CliffordK]

I visited a small museum at Windermere that had a collection of electrically powered boats that were very popular in Edwardian times although that said it was rare to find any surviving craft because of the destructive effect of leaking acid.
This is something to consider if using Lead acid batteries.

You might consider a "Dry Cell" battery.  Odyssey makes dry cell lead based batteries.  They are a little bit on the spendy side, but they are good batteries.  Perhaps the AGM batteries would work too.  Even Optima batteries.