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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

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  • \$\begingroup\$ How much is 'quite a lot' of current, and for how long do you expect the motor to be 'stalling'? What is the Ah rating of the battery? \$\endgroup\$ – Bruce Abbott Dec 26 '18 at 2:27
  • \$\begingroup\$ Chien Hao Tan - Can you provide a link to the manufacturer's datasheet for the battery? And, ideally, for the motor too? \$\endgroup\$ – SamGibson Dec 26 '18 at 3:00
  • \$\begingroup\$ Usually, if I have a concern about whether the current is acceptable, I would review the datasheet for the battery to see if it has any guidelines about maximum current. I have seen some lead acid batteries that have such. But quite a few don't. Barring that, I can tell you that a typical automotive starting battery can supply at least 100 Amps, or maybe much more in some cases, for 10 or 20 seconds. Unfortunately, construction details of lead acid batteries vary quite a bit. Flooded starting batteries, deep cycle batteries. Gel cells. AGM batteries. They all are different from each other. \$\endgroup\$ – mkeith Dec 26 '18 at 3:01
  • \$\begingroup\$ When you supply the links to the battery and motor, please put them in the body of your question, not in the comment section (in other words, edit your question, don't answer in the comments). \$\endgroup\$ – mkeith Dec 26 '18 at 3:03
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    \$\begingroup\$ 12v lead acid batteries used on trucks are capable of providing over 1000A - big truck engines need a lot of power, so check out your battery details and I have seen and dealt with 6V & 12V batteries but not 18V ... is your battery made from smaller nimh or li-ion cells? \$\endgroup\$ – Solar Mike Dec 26 '18 at 5:11
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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
  • Cost

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.

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    \$\begingroup\$ In Winnipeg it often needed more than 30 seconds in Feb if it was <=-30'C and an old car with a lot more piston friction and a >50% weaker battery @ -30C than 0'C. The better solution was often a battery blanket heater, engine block heater, inline water heater or if desperate diesel starter and booster cables and if none of the above, and no CAA nor a battery charger, a toaster and diode with AC extension cord. In Churchill at -50'C the Govt vehicles just kept running all day. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 26 '18 at 14:31
  • \$\begingroup\$ It's 730 CCA at more like 7 volts. On the upside, the numbers only get better when the battery gets warmer. I've seen lead-acids burn off their own terminals when starting an engine. \$\endgroup\$ – Harper Dec 26 '18 at 21:45
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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.

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    \$\begingroup\$ Have you ever seen an 18V battery with a CCA rating? Come to think of it, I don't think I have ever seen an 18V lead acid battery. I wonder if OP has 3x 6V battery in series? \$\endgroup\$ – mkeith Dec 26 '18 at 5:03
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    \$\begingroup\$ No. But they exist on Alibaba. That does not change my answer. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 26 '18 at 5:25
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    \$\begingroup\$ Just make sure the Alibaba ones are not 24V batteries with 3 cells already failed before you buy them ;) \$\endgroup\$ – alephzero Dec 26 '18 at 10:35

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