Mixing battery types.

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Sarnia

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I am considering increasing my battery banks, all lead acid, which are 2 x 180 amp/h + a 75 amp/h starter battery.
I would like to increase my domestic from its current 360 amp/h by adding 2 x Trojan T105 6v x 225 amp/h in series to give me 585 amp/h.
Is there any reason why they cannot be mixed in the way outlined? my instincts are that a lead acid battery is a lead acid battery in whatever guise.
Opinions and expert advice welcome .
 
You're right - if they're the same battery chemistry, there's no problem. Some folk rooted in the mists of time may suggest that different capacity batteries shouldn't be combined, but this is bollox.
 
sarabande said:
I have an uncomfortable feeling that it makes a difference if the + and - are taken off the same end of the series, or off opposite ends.


I'll draw a diagram when I wake up to show what I mean.
Click to expand...

If the connections are sound, and the cables suitably substantial, the real-world effect of what you suggest is neglible. It's a desirable, rather than a must-have. There are worse things to worry about.
 
for this relief, much thanks. :)


Blame my excellent Physics teacher of nearly sixty years ago (Mr Gibson) for instilling a lot of theory which has long been dormant..



My only Q, pvb, is what if the batteries, though of the same type, are of different capacities. ? Does the "same end" and "opposite end" theory still hold good ?
 
I am now definitely more uncomfortable having seen the ultimate connection diagram on that link. I shall have to buy yet more battery cable and terminal connectors for my proposed three battery bank. :)

thanks Poecheng.
 
Research by Lifeline Batteries has proved that a bank can be made up of different capacities and that the life of the batteries was not compromised. This goes against most previous guidelines, but Sabre yachts, who use Lifeline AGMs, regularly ship with different sized batteries to be able to get the maximum capacity from all the available space. But Lifeline say they must be all the same make and the same age. ​
 
sarabande said:
I am now definitely more uncomfortable having seen the ultimate connection diagram on that link. I shall have to buy yet more battery cable and terminal connectors for my proposed three battery bank. :)

thanks Poecheng.
Click to expand...

The proposal on the Smartgauge site is for geeks. As said before, if the connections are sound and the cable substantial, there'll be no discernable benefit from hugely complex wiring arrangements.
 
As said I don't think the wiring arrangement is at all significant if the wiring is stout (at least for long life) and it is only for low current domestic service anyway.
What would be desirable is to have an isolation arrangement so that on occasions you can compare the performance (time to discharge) of the different batteries in the string. With parallel batteries any faulty battery will drag down the performance of the whole. good luck olewill
 
Battery are mind blowing , we depend on our batteries as we live off the hook nine month of the year , I don't pretend to understand them but some years back I spoken to three different batteries company and there all said the same , your can't mix gel and lead acid , regarding ah it don't matter if you have two 100 ah and one 120ah it ok to mix , regarding mixing old batteries with new once , not a good idea . We have five house and an starter battery , and if one of the house packs up we just remover it until we down to three batteries , then we replace all five .
www.bluewatersailorcroatia.webs
 
I assume that if batteries are in parallel but different capacities then as smaller capacities discharge their voltage drops more than the higher capacity ones which then provide more of the power, so you would use the capacity of the larger battery. Not sure having two six volt batteries in series in parallel with twelve volt ones would do the same as they may not behave exactly identically. If one is discharging faster than the other it may drop off enough so that the above does not happen.
 
so far the only technical explanation has been from Smartgauge, plus some commentary from clearly experienced forumites.

OK, the difference in wiring strategies may be marginal, but the idea of losing battery power for the nav kit in pretty thick fog a mile from Plymouth breakwater (again) does not appeal. That margin of battery exhaustion after a 200 mile passage might have been just enough to get inside.


I would really like to know why it doesn't matter how you connect them, and if it does matter, again why.
 
sarabande said:
I would really like to know why it doesn't matter how you connect them, and if it does matter, again why.
Click to expand...

But it does matter how you connect them - all the connections need to be soundly made, and all the cable needs to be hefty enough to cope with anticipated currents without causing unnecessary voltage drops. After that, complex wiring arrangements might succeed in raising the charge voltage of some batteries in a bank by a few millivolts. Sledgehammer and nut scenario. If there are 4 or more batteries in a bank, having the main positive and negative connections at opposite ends of the bank is a simple and worthwhile measure.
 
pvb, I take your point about having the right cable dia, and making sound connections, both of which are "best practice" and which many people ignore.

The strategy of "opposite ends" connection does seem to be better than "same end", but the Smartgauge reasoning does show a drop in voltage extracted for a poor design. Now, I agree that it really shouldn't make any practical difference when the bank is properly charged and maintained, but if there is any chance that a poor connection strategy will drop one or more batteries in a bank down to an unacceptable level (which I will not attempt to define :) ) on a long passage, then I don't want to arrive at that situation if it can be avoided.

I'm not challenging your obvious experience and knowledge, but I just feel there is an optimum run for cables on a multi-battery bank (same types, of course), and want to find out what that is, and why.
 
This thread is a learning curve for me, I had understood the benefits of the smartgauge diagrams and connect as option 2 that is + and - from opposite ends, I must come down on the comments from pvb particularly in connection with proper sized cables and good clean connections to the battery terminals.
Thank you all so far for your contributions and technical expertise.
 
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neilf39 said:
I assume that if batteries are in parallel but different capacities then as smaller capacities discharge their voltage drops more than the higher capacity ones which then provide more of the power, so you would use the capacity of the larger battery.....
Click to expand...
The water tank analogy helps to explain what is happening.

Two water tanks (batteries) of different size (different capacities) and joined together by a common outlet pipe (cable), will have the same head of water (voltage). The smaller tank must have the same height (same nominal voltage) as the bigger one or I will overflow (overcharge). When the tap is opened they must discharge at different flow rates (current) to keep the head of water (voltage) the same. They will both lose the same percentage of their original capacity. The same works when filling (charging), so two different capacity batteries can happily work together.

Two take the analogy further to explain why different types and different ages of batteries don't work well together you have to consider how the batteries charge and discharge their energy. To do this imagine many smaller tanks (thick battery plates) within each tank (battery), each connected internally by small pipes EDIT: that get smaller and smaller between the tanks. When charging the first internal tank fills first (surface charge) and water slowly percolates to the other tanks (deeper into the plates). This helps to explain why when a battery is discharged quickly it's voltage falls (the first tank) and when the discharge stops the voltage rises as it is filled from the inner tanks (deeper plates).

This really needs an animated graphic to make it easier to understand.

So apply this to batteries of different types and age. They will have slightly different terminal voltages at different states of charge (so the height of the water in the tanks will be different) and their internal resistance will be different (the size of the small pipes between the inner tanks will be different). So when charging or discharging stops then the tank (battery) with the lowest internal resistant and/or higher nominal internal voltages will top up the other tank (older battery) to equalise the heights.

So mixing two batteries by age or type means there will be unequal load sharing as one battery is always trying to top up the other one. This will shorten its life cycle.
 
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sarabande said:
OK, the difference in wiring strategies may be marginal, but the idea of losing battery power for the nav kit in pretty thick fog a mile from Plymouth breakwater (again) does not appeal. That margin of battery exhaustion after a 200 mile passage might have been just enough to get inside.
Click to expand...


Scilly Scarabande! :rolleyes:

You know perfectly well that you had a 'gold-plated, TriService-issue, recently-calibrated analogue precision navigation system' fitted close to hand for use as a get-you-home reversionary system, which functioned exactly as advertised on the tin when the requirement emerged.

The only difficulty you had, in that specific circumstance, was finding enuff sugar for the 'reversionary navigation system's coffee...... which is what it runs on!

......Oh, and a modest supply of IKEA 2B pencils, and the occasional HobNob.
 
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