Problem with lead acid battery

I have 150W PMDC motors which run on 24V. I connected two lead acid batteries of 12V and 7.5AHr in series to drive them. But the problem is that after running the motors for about 5mins, voltage of one battery drops to 9V when motor is rotating and it again shows 12V(its fully charged voltage) when motor is stopped. Every time the same battery shows the problem.What could be the problem?

Sounds like a weak battery that has reached the end of its useful life.

What is the battery's exact voltage? It should be about 12.6V if fully charged.

I agree with tcmtech, it sounds like a weak battery.

p.gholap93 said:

voltage of one battery drops to 9V when motor is rotating and it again shows 12V when motor is stopped.

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Old 'dead' batteries have high resistance. Their voltage (under load) drops but with no load the voltage goes back up.

I concur - duff battery.

However, if you're putting batteries in series like this they need to be identical batteries, and both brand new.

So don't replace just one of them, or you're asking for problems again.

Yes It sounds like you have hairs in one of them batterys.