Fastest charge, 100AH x 2 or single 200AH battery.

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jonjo

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Hopefully pure science will offer a clear answer here.

Given a 55 amp charing source could I expect two 100 amp hour 12v batteries to charge faster than a single 200 amp hour battery?

The reason for this question is that I am juggling options to upgrade my domestic battery capacity and one benefit I am hoping for is that the higher capacity will soak up alternator output faster = fewer engine hours at anchor. At present when the domestic battery is down at 60% charge I notice that after 20 minutes the charge rate begins to fall off quickly.
 
Surely two 12 volt 100 Ah batteries wired in parallel are the same as one 12volt 200 Ah battery, so there would be no difference.
 
I reckon the single battery will be very slightly more efficient. Dunno why. Economy of scale?? It should be physically smaller since there's only one box?

You could argue it will be more reliable too compared to two batteries permanently paralleled. Cos if one fails it takes out the other.

If all else fails toss a coin.

Mind you I wouldn't want to carry a 200Ah battery. Did my back in carrying out the old truck battery on ours.
 
I think pure science agrees with alan_d.

A smart alternator regulator may be the way forward.
 
The charge rate is all a question of charge percentage.
So if you had 200AH at 80% charge after 24 hrs of use.
This might mean that had you a 100AH battery it would be down to 60% charge. Each having had 40AH removed.
So for a given limited engine run your ie 1 hr 200AH battery might take on 20AH and move it from 90% to 90% charged.
For the same engine run your 100AH battery might take on less charge because it is a smaller battery but more because it was more discharged.

The relationship of course is not linear and I think the 200AH battery will take on more charge from 80% than the 100 would take on from 60%. The charge of course will taper off but tend to be higher for the 200AH.
200AH being the same as 2X 100AH.
This is because the inherent voltage of the battery rises from low at discharge to higher at near charged very quickly.

I think you will be better off with the double sized battery. However I think when you come to buy the batteries that price will dictate what you bu8y. ie 100AH will be more common so cheaper than the 200AH. And as said will be a lot easier to manage lifting. Further more you might benefit by fitting those terminal screw down switches so that you separate the 2 batteries so ensure one is not dead pulling down the other as they approach end of life. Of course having 2 batteries is going to increase your battery costs. They may last a little longer (but not twice as long) being less discharged but will also die of old age as well as abuse.

In the end for minimum engine run time you need a smart charger. But This will only improve the charge by a factor which might not justify the cost. Likewise added battery capacity. Much depends on how much you will use the boat away from shore power and how much you might voyage under engine. Over to you to decide olewill
 
Yes there should be no difference if they are both in circuit. In a practical sense all that two batteries would do is give you the possibility of isolating one for insurance if the other goes flat.
 
but your two 100's might have more plate surface area than a single 200. surface area means more chemical reaction so more amps both in and out...

agree with boatmike tho, and VicS.
 
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Surely two 12 volt 100 Ah batteries wired in parallel are the same as one 12volt 200 Ah battery, so there would be no difference.

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Is it that simple? What factor progressively slows down a battery's rate of charge absorption particularly in the +80% charge range. Whatever this factor is, will it kick in the same way with two smaller batteries?

I suppose at the end of the day a battery is just a plastic box holding a surface area of lead plate dangling in liquid acid. Given the same make and cycle grade I am going to assume the number of batteries is not a factor in charge rate.

Anyhow having discussed all this with Malcolm the Hat at Haslar it seems a 200AH battery is the best upgrade option for me because the Bosch replacement he has in mind is the same size except it is 50% wider. Fitting practicalities now outweigh scientific factors!
 
Thanks everyone, when I made the original post I was viewing a battery as a mysterious black box. However as many have said at the end of the day it just comes down to plate surface area and victorian vintage chemistry,
 
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There is no discernable difference between 1 x 200Ah and 2 x 100Ah. You still have X number of amp hours to replace.

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Do you know for sure that double plate area in the 2 x 100Ah has no effect in charge acceptance, espcially in the absorbtion phase? Anyone have anough money to buy some batts to test this? I could take them off your hands once you´re done...
 
I've only been in the battery business for 20 years! I use the word "Discernable". You are more likely to get a slightly longer recharge time due to the resistance in the battery cabling than that casused by plate differences.
 
Good answer /forums/images/graemlins/grin.gif Sometimes there are so many opinions it´s hard to know who is waffling and who knows what they are talking about.
 
You wouldn't believe some of the things I hear on my stand at the boat show - many people are (quite sadly) completely confused by overstated claims, "expert advice" and wholly incorrect magazine articles they read. Marine electrics aren't a black art but there is a huge amount of over complication and bullsh*t associated with it! Using the Keep It Simple principal is always the way forward (and usually - the cheapest)! /forums/images/graemlins/wink.gif

All the best - James
 
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