Can I connect 12V Battery and 24V battery in series?
short answer, Yes.
long answer, yyyyyyyyyyeeeeeeesssssssss.*
* Keep in mind because of kirchhoff's current rule, that the current flowing through all the series batteries will be equal. So the most amps you can safely draw will depend on how much current the weakest battery can deliver.
* Also keep in mind that recharging rechargeable batteries in series is not a good idea without some way to keep the batteries "balanced." If the batteries are unequal in ESR or capacitance (which they will be even if they are the same type and stuff) than charging them in series can result in one charging faster than the other, leaving one not topped off and the other pontenially overcharged. In the same way, one or more cells in the series can end up over discharged if they the battery packs are not perfectly matched.
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So as a really basic rule of thumb, you can usually connect batteries of different voltages but same capacity/current capabilities in series, and connect batteries of different capacity (mAH) and current capabilities so long as they are charged to the same voltage and have the same nominal voltage ratings.
When connecting batteries in series or parallel, make sure that they are fully charged first, so that the are at the same voltage level and charge level, and when charging batteries in series (especially lithium batteries) make sure to use a balancing charger that detects the voltages on each individual cell so that one does not end up overcharged.
Putting batteries in parallel is a bad idea, unless they are practically of the same batch and charge history. Minute variations in output voltage cause large circulating currents, unless the battery's ESRs are significant, or you add a current sharing resistor.
Interesting. Understandably it is not good practice, but I figured if they are charged to the same voltage it should be OK as they will share the current equally. I have a flashlight that uses 4 18650 batteries and I make sure to charge each one fully because the flashlight simply connects them all together in parallel. I have never had a problem with it, I luckily have never managed to insert a battery backwards. I figure it is probably best to leave them in parallel once connected if you can so that they stay matched together.
I figure it is probably best to leave them in parallel once connected if you can so that they stay matched together.
That's the best way to make a decent job of a bad idea for sure - just don't do it, if you can avoid it....
Its quite common in RV and solar applications to parallel deepcycle cells, but, as I suggested, they are usually "matched sets"
Yes you can do.
But the voltage will be added 12v + 24v = 36v
But why do you want this much high dc voltage??