I have a motor I wish to drive with an 18V lead acid battery. The motor can draw quite a lot of current when stalling and I am worried of overdischarging the lead acid battery. Unlike LiPo batteries with have a maximum current rating, the lead acid battery only stated the "initial current", which is used for charging. The label stated not to short the battery. Hence, may I know what/how to find out the safe current to draw? How will the battery fail if I draw too much current (explode/lifespan decreased/?)? Thanks
Lead acid batteries are fantastic at providing a lot of power for a short period of time. In the automotive world, this is referred to as Cold Cranking Amps. From GNB Systems FAQ page (found via a Google search):
Cranking amps are the numbers of amperes a lead-acid battery at 32 degrees F (0 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery).
A car actually doesn't need 30 seconds, normally only a few seconds to start, except in very cold weather or other extreme situations. But 30 seconds means you could give it a few tries before the battery (if it started charged and in good shape) becomes a problem.
Picking one battery at random from Sears DieHard, I found 730 CCA. That's 730 Amps at 12 V. That is a LOT of power.
The end result is that you have to find the right battery combination of:
- Max. surge current or CCA
- Total capacity, typically in Amp-hours
- Size & weight
An 18 V lead-acid battery could be a tiny little thing. Or it could be like a typical car battery, only 18 V instead of 12 V.
If you are looking at a one-off or small production run you may be better off leveraging car battery technology - mature, inexpensive, easy to replace - rather than 18 V or something else more unusual. But that only works if the size & weight parameters work for your particular project.
The max safe current is the CCA rating for 30 seconds max and 30 second intervals.
Exceeding this may warp the plates of batteries, boil the electrolyte and with sparks create a safety hazard.
A car starter will not exceed this rating when sized properly and the voltage will not drop below 7.5V, which is the criteria used for the CA and CCA test. This the power dissipated internally is (12.5V-7.5V) * CCA = Pmax
e.g. 5V drop x CCA rating of 800A = 4000 Watts of heat for 30 seconds can supply 7.5V * 800 = 6000 Watts of power to the load.
This is the maximum "safe" power transfer.
The starting current for a DC motor is 8~10x the "rated current", and is also called the "stall current". This may be calculated or measured with a voltmeter.
Obviously, this ages the battery faster with frequent max CCA tests and also if the left in an undercharged condition < 11.5V but cars do not typically draw the max CCA rating and the lights do not dim to half power, but if they do then battery may be weak, badly sulphated or simply have high ESR.