Quote Originally Posted by Kevin22 View Post
In series for 24v will not pull equally from each battery. As the lead acid gets low (below 12v) it will pull more amps from the lower voltage battery than it will the lithium, draining it even faster until eventually the lead acid is pulled down so low it gets damaged. You wont know there is an issue because the lithiums constant
13.1-13.3v will continue to produce 23v+ when the lead is toasted and the motor will continue to work somewhat well.

Never ever ever mix battery chemistry, from AAA up to deep cycles....

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Please enlighten us as to HOW the different amounts of current will be "pulled" from each battery. Hint - it doesn't. I mean are you saying that if you put an amp meter in series on the positive lead of each battery you will get different amperage readings? Won't happen in theory nor in reality.
Now if you were to let either battery draw down too far either could be damaged, but remember he is looking at it as a back up, in case something were to happen to one of his batteries. That's what he said. He's not using it daily and draining it down past it's ideal usability.

Of course it's not an ideal situation to "mix" battery types, but in this situation would be viable backup as @bricks suggests.