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I am interested in several things: What happens right when I connect them? How does this behavior affect the batteries, both short term and long term?

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  • \$\begingroup\$ Sorry for the poor quality of the photo :( \$\endgroup\$ – Mads Skjern Apr 22 '14 at 19:53
  • \$\begingroup\$ Can you explain why you want to do that? That may help in writing satisfactory answer. \$\endgroup\$ – Kamil Apr 22 '14 at 20:25
  • \$\begingroup\$ @Kamil: I don't want to do that :) I am trying to plan a stand alone system, and there is a lot to learn. This is just one out of many small things I need/want to understand. Every time I resolve one such question, my overall understanding improves. And well, maybe one day, one battery is discharged, and one not, and then the question is very relevant. \$\endgroup\$ – Mads Skjern Apr 22 '14 at 20:50
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    \$\begingroup\$ So you need some battery monitoring/cut-off system :) And maybe battery charger with balancing. I smell solar or wind energy :) \$\endgroup\$ – Kamil Apr 22 '14 at 20:54
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  1. Short-term, this isn't a problem. You'll end up with ~24V powering the load.
  2. Long-term, this is a bad idea. Once the half-charged battery gets drained, the full-charged battery will now be at half charge and will start running the drained battery in the reverse direction. Lead-acid batteries hate to be deep-discharged. The lead plates will corrode and you'll lose capacity on them permanently if not destroy the battery entirely.

To prevent the second battery from running backwards or even being deep-discharged, make sure you balance the batteries before connecting them in series and running them to the load. A poor man's semi-decent attempt to balance them would be to just put both batteries in parallel for a long time to transfer some charge from the full battery to the half-charged one.

Another option is to always monitor the half-charged battery to make sure that you never deep discharge it. You'll have problems charging it as well though. Upon charging, you'll likely end up over-charging the full battery which is just as destructive, but in a different physical mechanism.

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Right when you connect them, current starts to flow through both batteries and the bulb. Nothing out of the ordinary happens. The half-charged battery will be depleted first and its voltage will fall. This is very similar to having a bad cell in a battery.

The real problem you will have is charging them when connected in series. You can't use a constant-voltage charger because it assumes the same voltage across each cell, and a constant-current charger will overcharge the half-full battery before the other one is fully charged.

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Best thing is to charge the weak battery before using it, so the stronger one doesn't charge it in reverse. Otherwise, you can put them in parallel for double the ampacity at 12 volts, and use a voltage doubler to drive your 24v load. Note: leaving lead-acid in a low charge condition for a long time is one of the worst things you can do to it, as the plates will sulfate and develop a high internal resistance. I live off-the-grid myself and have replaced several batteries for this reason.

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