Is it me or do some theoretical people seem to have trouble with real world use of standard technology. In the real world it is quiet usuall to put more than one battery in parallel, in the real world this is not a problem. You see the melt down situation MH spoke about cannot possibly happen, the reason for this is with a discharged battery the impedance is high, the more discharged it is the higher the impedance is, this battery with regard to melt down situations is in practise approaching an open circuit, there will be heat dissipated because yes there will be current flowing, but the low impedance and thermal mass of the fully charged battery which has a very low impedance but not as low as the wire will get hotter, infact a battery charger or alternator only manages to recharge a lead acid battery by elevating the charge voltage to above the battery discharge voltage because of this, otherwise you may die of old age waiting for the battery to charge fully. It does not take a lot of internal resistance for the source battery to lower it's terminal voltage enough for the target battery to take little current. If you place a charged car battery in parallel with a discharged battery, how long do you think it will take for the discharged battery to take half the charge away from the charged battery, well i can tell you that you could probably go down the pub, get pissed, sleep of the hangover and it may just be almost there barring losses, this i must say is a world apart from the meltdown situation MH spoke of, and this is scaled up to any size of battery because the thermal mass also increases with AH, now i am not saying this is good practise, anyone in the practical world knows not to do this. So to recap, you can always connect lead acid battery's in Parallel this is 1/never. I have been pushing this because this is very basic practical torch battery/ bulb stuff.
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