# Charge at 24v and discharge at 12v for battery system

Is my thinking correct.

Below is a simple diagram of the concept I want to check.

Is this: 1: Possible 2: Feasible? I.E are there any concerns or foreseen issues with this design?

Edit:

• Why would you want to short out the top left-hand 12V battery? Also, why would you want to short out the top right hand battery? Is your concept about melting batteries or starting a fire? – Andy aka Mar 3 '14 at 12:17
• It would need a few switches to avoid the short circuit Andy noticed, and some batteries don't like being connected in parallel (the weaker one tends to leech charge from the stronger) but in principle it COULD be OK. As long as you realise it can EITHER charge at 24V OR discharge at 12V and you can guarantee there is no way to (accidentally) set the switches for both. That could start a fire. – Brian Drummond Mar 3 '14 at 12:27
• Short 1: RHS battery wire from red terminal to 12V output then to red terminal of other battery and finally to black terminal of RHS battery: So in effect directly from red to black. There is a similar path for the other battery too. – Warren Hill Mar 3 '14 at 12:34
• The diodes do NOT help. Note very carefully how much (and what polarity) voltage is across each diode. Do you see why your "12V output" will actually have 24V (minus 2 diode drops) across it? – Dave Tweed Mar 3 '14 at 13:33
• If you have never seen a 12v battery boil, burn, or explode, go spend 5 minutes on YouTube before you get the tools out and start wiring anything up. – John U Apr 3 '14 at 8:06

It will work fine as long as gray and brown wires are not present at the same time.

The diodes are redundant, they don't fix anything - they will prevent a short-circuit present when both green and brown wires are attached, but the output won't be 12V.

I don't see why you simply won't keep the 12V parallel connection of batteries and charge them from a 12V charger?

Let's not forget that the ideal charging situation is when each battery cell is handled by an individual charger. In a 12V lead-acid battery there are 6 cells, each with 2.0V nominal voltage. Ideally, the cell-to-cell links would be exposed, and one would use a 7-wire connection to the battery for both charging and discharging (with a charge/discharge controller). This is the ideal scenario and maximizes battery life. This is how electric cars and professionally designed battery packs are made (in contrast to cheap junk).

So, even the situation with a 12V battery used as a unit is less than ideal since there's 6 cells in series, without individual cell management. By connecting two of the batteries in series, you're making things even worse.

On the second diagram why don't you add the fifth diode between battery A and B so that the power supplied from D4 & D2 doesn't short the 12V output/motor output due to the grey wire that connects the two wires D2&D4. I think the fifth diode can separate the and allow the flow of series to flow from negative to positive which is a normal flow of series connection. Not 100% sure but its just a suggestion.

• Hi and welcome to EE.SE. "Not 100% sure but its just a suggestion." is not something which would constitute a good answer. Moreover, the question is over three years old and already have an accepted answer. – winny Aug 16 '17 at 13:43
• Welcome to Electrical Engineering Stack Exchange. Please take the tour at electronics.stackexchange.com/Tour to get the most out of this site. Ditto, @winny. – SDsolar Aug 16 '17 at 16:38