Battery charging.

[doug748]

I have two 12v batteries one 110 Ah the other 60. The larger battery has been kept up with a small solar panel and I would like to give the smaller battery a boost, as it has not had a charge for 3 months or so.

If I connect + to + and - to - using booster cables. Then my theory is that the smaller battery will be charged the larger discharged. Job done.

Is this correct, will huge sparks occur and armageddon ensue when I clamp on the cables? I guess the little solar panel would then charge both (but would struggle) ? Could I nacker any on board electrical kit?

I have no engine available or electrical power. All thanks.

 

[davidej]

I should think this is correct until the voltage in the two batteries is equalized. However the voltage difference is likely to be small so the transfer may be slow.

 

[halcyon]

A lot depends on initial state of charge of both batteries, the output current and voltage of solar panel. When you charge a battery you need a higher voltage, over 13 volt, when you discharge a battery you get a lower voltage, so a battery 12.8 will fall to a lower voltage. This means the solar output voltage is set by the small battery, until it reaches the same charge as the large battery when they both take a charge. This is the principle behind the VSR.

Brian

 

[lw395]

The two voltages will be made equal.
At the moment of connection, there may be a spark as a high current flows due to the capacitance or surface charge and the voltage difference.
Then things will settle to a steady state.
If the big battery is resting fully charged at say 12.8V, then the current flow into the small battery will drag it down a bit and the small battery will charge very slowly. There will be a nett loss of charge. Because charging lead acid batteries is not 100% efficient.
The solar panel will charge both, but mostly the small battery.
The voltage is forced to be the same, each battery will charge or discharge according to its V/I/state-of-charge curves. The currents must add up to a nett zero.

How low is the small battery?
The most satisfactory thing to do might be simply switch the solar to the small battery for a day/week/????
Or take it home and give a proper charge.

 

[Sandy]

Why not just change the solar to the small battery for a week or so?

 

[differentroads]

Iw395 nails it.
I've had to do on my boat what you describe. And its no more dramatic than jump starting a car. it won't affect your electronic equipment. There will be a fairly small crack and a spark and the battery with the lower voltage will start to charge from the one with higher voltage plus the solar panel (while there is sun, of course).
The small battery won't get anywhere near full charge. But how much that matters depends on your biat and power needs. A whole different issue.

 

[doug748]

Great. Thanks everyone, job done.
The discharge on the bigger battery will do no harm and and the charge to the smaller will tickle it up a bit. I use zero power over the winter, so happy days.