Wikipedia:Reference desk/Archives/Miscellaneous/2020 January 20

= January 20 =

discharge tolerance of car battery
I know about deep-cycle batteries but this question is about ordinary car batteries. It's well-known that one should not deep-discharge a car battery as that messes them up. Sometimes it happens by accident though: someone leaves the lights on and you have to jump-start the car. My questions:
 * Just how bad is an accidental full discharge, like above? That happened fairly recently with a car that I sometimes drive (other people drive it too), and I'm not sure exactly how.  The lights weren't on and the battery wasn't very old, but assuming it was accidental, did that probably decrease the battery's starting ability?  Is there some common problem with batteries that can cause this to happen spontaneously?  That incident was some months back and it hasn't happen again since.
 * How bad is it for the battery to partially discharge it on purpose, for example to recharge a laptop through the cigarette lighter without the motor running? Counting inefficiencies, let's say about 100 WH (8 AH, or maybe 10% of the car battery capacity) is withdrawn.  This is somewhat related to my earlier question about off-grid power.  It would be an occasional thing, not repeated frequently.  Of course it would not be done twice without running the car for a while.

Thanks. 2601:648:8202:96B0:0:0:0:4FFF (talk) 23:15, 20 January 2020 (UTC)


 * As far as I know, fully depleting a common (lead bases) car battery is bad. For a new battery, it won't kill it but it will reduce the lifetime.
 * How bad depends on a lot of things and can't be said without more info. One of the things to check is the voltage of the battery, both in use and not in use. For a disconnected, charged battery it should be above 12.4V (Note that some batteries have a higher rated voltage, check the label/manual). If it is below that after charging, you should seriously consider replacing it.


 * Another metric is the voltage during a (significant) load (engine off, heater/radio/highbeams etc. on). If it drops significantly during that load (e.g. more than 2-3 V) then that is also a bad indicator.


 * Regarding the partial discharge, most car batteries can be discharged to a certain level (e.g. AGM batteries should usually not be discharged below 60% of their rated charge). If you stay well above this limit (as 10% would be), this should not be a problem. Again, read the manual of your battery!


 * If you consider doing this more often, consider mounting an additional (deep cycle) battery in the car that gets charged by the engine (as a camper-van would have). Then you can pick a battery better suited for a slower discharge and if you empty it out, your car still starts (and with an appropriate shunt installed you could even emergency start your car on the deep cycle battery) . Rmvandijk (talk) 09:45, 21 January 2020 (UTC)
 * Thanks. AGM would not be used for a car battery.  The 10% discharge would be very occasional, though I could imagine doing a smaller discharge (like 1%) more often.  The self-discharge of a few days is higher than that, so it can't be too bad.  I didn't realize even AGM was supposed to be kept above 60%.  Lithium ion is now economically competitive with AGM so I'd probably go for that instead. 2601:648:8202:96B0:0:0:0:4FFF (talk) 18:06, 21 January 2020 (UTC)
 * I don't understand what you mean by "AGM would not be used for a car battery". I wouldn't recommend you install an AGM battery in your car if it didn't come with one and the manual doesn't say it's compatible with one, since it's unlikely the car was designed to charge it appropriately, plus it's an added cost for no real utility etc. But AGM batteries are common in start-stop systems [//www.aa.co.nz/cars/owning-a-car/batteries/start-stop-system/], especially in high end cars [//batteryworld.varta-automotive.com/en-gb/efb-or-agm-which-battery-do-i-need] [//www.yuasa.co.uk/info/technical/agm-efb-explained/]. For costs reasons, and with the increasing appearance of start-stop systems, lower end start-stop cars may use the so called enhanced flooded battery [//www.rbbattery.com/articles/2019/4/29/efb-battery-guide-everything-you-need-to-know-about-efb-technology] we seem to lack any real article on. Nil Einne (talk) 08:12, 22 January 2020 (UTC)
 * I just meant AGM batteries are expensive and intended for deep-cycle applications. Of course you could use them in a car, particularly a hybrid car, but I was asking about the ordinary car batteries that are found in today's so-far still prevalent petrol powered cars.  2601:648:8202:96B0:0:0:0:4FFF (talk) 06:47, 25 January 2020 (UTC)


 * It's not too bad to discharge it. What's a problem is to leave it sitting in that state. Andy Dingley (talk) 19:15, 21 January 2020 (UTC)
 * Interesting, thanks, that's good to know. 2601:648:8202:96B0:0:0:0:4FFF (talk) 07:44, 22 January 2020 (UTC)