How many amps can a battery bank take?

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zefender

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I have two new 180 a/h batteries on the domestic set, plus another 90 for the engine. I have a (I think) 60/70 amp alternator. Is there a rule of thumb to tell me how many amps the batteries can take over, say 2 hours? For my ARC trip, I've worked out a daily power budget of about 220 a/hours. I have a duogen which should produce about 180 amps per day, nice and gradually - more if we're lucky enough to get plenty of wind and thus waterspeed. So I'll have a daily deficit and will need to run the engine a bit to top up the balance - but I don't have enough fuel for more than about 2 hours running per day. How many amphours might I expect to produce in 2 hours? Also, might there be a surplus which the batteries can't use but which other equipment could whilst the engine is running, like a watermaker?
 
buy a small solar panel (50w), and this would provide the additional power and minimise the use of the engine - after all it might be necessary to be able to use the engine and if you have used all your fuel, you will have a problem.

Have you changed your tricolour light for a LED - that will save 12 amp/hrs.
 
220 a/hours seems a lot of power drain, what are you running to draw that amount of power, Englander uses about 200a/h a day, but we are running two fridges, a freezer (through inverter) and fresh water pump lights etc. I would have thought the power consumption sounds a little high. Thats over nine amps an hour. Your alternator, with a smart regulator, will whack in 60 amps for those two hours, unless the batteries are pretty well charged from the duogen. Which I would have thought they would be. Have you thought about solar panels? Also It might be prudent to have a small petrol genny, as an emergency backup, they are cheap and you may find it useful at anchor in the caribean. Coupled to a smart charger, which the genny will run (be careful about the choice) When you are running the engine for charging the batteries, that is the ideal time to use a watermaker. I would also say any surplus to requirement leccy could be used for the same purpose.
 
I have a small 40w solar as backup but realistically this will only give me about 25 amps per day. yes, I've changed the tri-light for a LED. Using the engine for 2 hours every day will use up 60% of my fuel, so I should have 95 litres left.
 
Might think to carry some extra fuel, just in case, if you have room, in the bilges?? Two 20 litre cans?
 
Hi Colin

Yep, it is a lot and maybe I've been a bit conservative. Obviously main power users are the fridge and the autopilot (excluding the claimed 8 amps for the watermaker, though actually this seems to be nearer 10 for the first 10 minutes). The autopilot can be adjusted to be less pernickety about the course and this will save quite a lot of power. The fridge (in hot weather) might need to stay on for much of the day/night. I assumed 18 hours. The LED trilight reduces power to about 11 amps per night. But once I add on VHF, SSB (receive) and other instruments as well as water/shower pump etc, music, sat coms etc it's surprising how quickly it seems to mount up!

The problem with a petrol genny is two fold. First, I'm on a Bav 40 - not quite the space you have to store stuff - and I'm running out! Secondly, I'd need to carry quite a lot of fuel to run even a small petrol genny - and I'm not that keen on having lots of petrol about, for safety reasons)
 
Bilges are only about 9 inches deep! I've already got to find space in lockers (god knows which) for 3 x 20litres plus another 25. Forward heads likely to be out of commission to free more storage (let's pray for a starboard tack all the way!)
 
Hey that's good to know. So if I can produce over 100amps in 90 minutes and these batteries are big enough to take it, I will more than cover my shortfall (assuming the Duogen does its stuff) and I can relax a bit on my dwindling fuel reserve. Thanks (phew).
 
You seem to have a bunch of comunications gear!!! I would be thinking about conserving energy, like turning stuff you dont need off! But I do understand your dilema, forget the genny, get another solar, find room for more fuel, or fit another tank, bag type. But I guess you ahve looked at some if not all of this.
 
The amount of amps the batteries can take will depend a lot on their state when charging. If they are at around 50% then they'll take a lot, if they're at 95% they wont take much at all, and how much surpless you have available will depend on this. Also note that the alternator output will vary with engine speed and you wont be getting a continous 60/70 amps from it unless you'äre running the engine at pretty high revs. A digital battery sensed regulator will help increase what you can get into the batteries, but running the engine for 2 hours isn't a particularly efficient way to top up batteries that are already in quite a high charge state due to the duogen.

A generator would be much quieter and more fuel efficient.
 
So, from what you're saying, it might be better to run the engine for a little longer but every 48 hours, rather than 24? This way, my shortfall will be about 80 amps or 22% of (domestic) battery capacity. Does this make sense?
 
[ QUOTE ]
Quick rule of thumb: max charging current is 1/3 of battery capacity, i.e. for a 210AH bank, max charge rate is 70A.

[/ QUOTE ]

That's a useful rule of thumb.

How much of that current is likely to be stored? ie if you put 70A in for 1 hour do you actually have 70A/h in the bag ?

(presumably not as they will get rather warm)

Thanks
 
Unsolicited and unwelcome perhaps:-

BUT given that it is not good practice to discharge batteries to 50% or less of their capacity (isn't 75% capacity more acceptable??) that would mean that you should have 440 a/h capacity as an absolute minimum, and probably a lot more....

Deep discharging and fast charging lead acid batteries (or any other for that matter) is not very efficient - you will probably need to add 25% extra power input to compensate. Anyway 20% of capacity is a much kinder maximum charge rate.

Even with deep cycle batteries I suspect you'll get a very short life out of your setup - and will have to top the cells up regularly..

But you didn't ask that did you?

Might be worth looking at the whole picture again - battery capacity / charge rate / consumption.

Sorry.
 
Re: Unsolicited and unwelcome perhaps:-

Well not really because although I (have budgetted for) using 220 amp per day, the Duogen will be topping up most of this throughout the day, so my shortfall will only be about 40 amps each day, leaving some 88% of the battery charge unused. So, even if I do run the engine every 2 days, it should have some 78% charge left when I turn the donkey on. Unless I'm wrong of course!
 
Re: Unsolicited and unwelcome perhaps:-

Haven't been through your sums in detail, but my perception is that you won't have too much of a problem. Clearly, you will be able to judge your consumption of charge and hence fuel as you go, and make adjustments accordingly.

My main point is to bear in mind that the batteries will not be 100% efficient as they will use/lose some energy themselves. Moreso as they get older, so you may need need to put in more charge than you take out.
 
Tracking battery condition...

I'd think one of the most important things you should fit is a good battery condition meter. The Xantrex one (updated version of the highly-respected Link 10) is good, as is the BEP DCM600. Either of these will keep you advised of exactly how many amp-hours you've taken out of the domestic bank, so you'll know the best time to start recharging. They'll also show you how quickly the charge is being replaced, allowing you to judge when to turn the engine off.

You'll also need a smart regulator for your alternator, but even with this you won't see a 60A charge into a 360Ah domestic bank unless the batteries are fairly low. The most cost-effective upgrade to your electrics would be to add more battery capacity - this would make charging more efficient and would reduce the depth of discharge for a given consumption. Reducing the depth of discharge will extend battery life; however, having said this, you might find that the most fuel-efficient charge regime during your trip is to let the batteries drop to say 65% of capacity and then recharge them to only 85-90% of capacity.

The fridge is going to be a big consumer of power. You might see whether it's possible to increase the insulation around it, which will help to cut consumption. Also, if it's an air-cooled condenser, make sure there's a good supply of fresh air to cool it. If the condenser is hidden away in a badly-ventilated bilge, it's sometimes worth adding a small 12v cooling fan (computer type) to ensure a flow of air to the condenser - wire it so it only runs when the fridge is working.
 
No, Snowleopard was referring to the maximum charge rate your battery bank can take. Your problem is the output of your alternator. It seems underpowered for your requirements. You have 360 ah capacity, which is less than twice your daily usage. This is very tight, but I think your usage figure is very high and unlikely to be required. You have a 60 amp alternator which will actually be putting out about 40 amps, and this is only when the battery bank acceptance rate is high, when it is less than half charged. Even with a good multi-stage regulator your rate of charge will fall off as the battery becomes charged. I think you might achieve 30 amps in an hours running when taking your battery bank from, say, half full to about 80% charged. Getting beyond 80% charge takes a long time. Dropping to below 50% will eventually kill your batteries.This is why you should design your battery bank capacity to be three times usage - only the capacity between 50% and 80% full is useful capacity. When I cruised warmer climes I had 240 ah capacity and a 60 amp output alternator. I used about 90 ah a day and needed nearly 3 hours running per day to replenish it. The price you pay for cold beer!
 
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