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[hopeitfloats]

if i connect a 6v 12ah in series with a 6v 4ah battery to give 12v will the current draw come equally off each battery or will the 4ah one drain quicker. sorry if it sounds a dumb question and i think they will draw down equally but just need to confirm or throw my theory out the window.

[roycv]

Hi Hope it floats, the current will be the same I think it is Kirchoffs that says that.

The voltages across the batteries will be marginally different until the 4 Ah battery is discharged (i.e. the voltage drops to zero) then you have the possibility of the 4 Ah battery being driven negative.

Thats the theory, I would not do it in practice unless you are needing 12 volts for a short time.
best of luck Roy

[pugwash]

Hi Hopeitfloats,  When i had my motor-sailer which had 110 ah batteries 4 in number when replacing them  we had to
fit the same ah or it was not good for the batteries - so said the man in the chandlery. But I think it has
something to do with the charging cycle which we did from the boats diesel - you will be charging them
individually so they shouldnt take and harm

[Niall]

I've aways heard that it can be dangerous to wire different batches of the same size cell, much less different size cells together.

What will happen is the 4Ah battery will run flat then the 6Ah battery will short into it, possibly causing a fire.

[maninthestreet]

If the 4ah battery goes flat, it's internal resistance will be much higher than still-charged 6ah battery it's in series with, so I don't see how a short-circuit can occur.

[AberdeenAngus]

Hopeitfloats.

You will, in effect have a 12v 4ah battery. You are right in saying the current draw will be equal of each battery. This, of course means the smaller cell will flatten first.
This should work fine until you have discharged the smaller cell, at which point it may suffer damage.


Pugwash
Your 4 batteries will have been mounted in parallel. In which case it's best to use the same type and even batch otherwise slight differences in chemistry will mean they discharge through each other.

[hopeitfloats]

it might be safer to buy another 10amp battery i think.

[hopeitfloats]

that should have read 12amp