Battery Charging Time

Submitted: Tuesday, Mar 04, 2003 at 19:56
ThreadID: 3659 Views:3049 Replies:4 FollowUps:1
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Hi all,

I've been trying to find out charging time's for car batteries without much success, hoping someone on this forum can help out !

I need to charge an Exide Extreme Battery N70EX 80 A/H with an 240 Arlec battery Charger ($30 Kmart version purchased 10 years ago !). The Arlec Battery Charger specs quote 5A RMS (12V 3.5A Continous).

The Battery is brand new (off the shelf).

How long do I need to charge the battery for ?

What is the lowest voltage I should allow the battery to get to before causing potetial damage to it ?

I have an Electronic Multimeter, does the voltage reading of the battery give me an inidication of the state of charge ? (current voltage reading is 12.93V - there is no load on the battery).

Phil P
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Reply By: Bob Y. - Tuesday, Mar 04, 2003 at 22:09

Tuesday, Mar 04, 2003 at 22:09
Phil, just come off the Exide site( and the only info I noted for recharging was "fully discharged auto battery using 10 amp charger, will take 8 - 10 hours @ 80*F to reach full charge."

I'm not really up to speed with theory of these black boxes, but to check charge you would need a hygrometer, to read specific gravity. With no load voltage @ 12.93 feel it wouldn't be too far off full chg. Those Arlec chargers aren't much better than trickle chargers, so you could give it a fair run, without damage.

Just forget the lowest voltage, for long life, but am sure it was mentioned here before. Ozi mentioned it a number of times. Think some people use a percentage, say 10% drop. Hooroo...
AnswerID: 14452

Reply By: Eric - Tuesday, Mar 04, 2003 at 23:26

Tuesday, Mar 04, 2003 at 23:26
Your battery has a capacity of 70 amp hours. so if it is completely flat it will take 70 hours at 1 amp to charge it. Simple. If you charge at 2 amps then 35 hours, etc, The maximum voltage you should charge at is 14.8v any higher and you will start to damage the battery. the minimum voltage is not a big issue it is the time the voltage is low is the problem, ie. if you flatten it charge it a.s.a.p. A battery left flat will sulfate the plates making it very dificult to charge. 12.6v is the normal voltage under load. The voltage is not a good indication of charge, mental arithmatic, how much load how many hours? is the best way. Eric.
AnswerID: 14458

Follow Up By: Rob - Wednesday, Mar 05, 2003 at 00:14

Wednesday, Mar 05, 2003 at 00:14
A 70 AH battery - should deliver a current of 1 amp for 70 hours.
Or, in theory, 70 amps for 1 hour - but unlikely, unless it is designed for this.

Assuming a 100% efficient battery, charging for 70 hours at 1 amp
would then allow 70 hours discharge at 1 amp.

Unfortunately, batteries are not 100% efficent e.g. they will get warm during charge. Thus you would have to charge for longer than 70 hours (at 1 amp) to replace the charge you had taken out of it.

Typically, a lead-acid battery is around 70% efficient meaning you would have to put around 42% 'extra' charge into a battery.


FollowupID: 8703

Reply By: Rod - Wednesday, Mar 05, 2003 at 14:45

Wednesday, Mar 05, 2003 at 14:45
There's an easy way to calculate how much charge is left in your lead acid battery. Easy yes, accurate may be no.

See the table at :

Note that you will need to disconnect the battery from load and charger and leave stand before taking voltages. Ideally you leave them for 2 days but that is rarely practical. If you monitor the voltage accross the battery terminals and disconnect the charger, you will notice the voltage drop off with time. Eventually the decline willl slow to a trickle.
AnswerID: 14496

Reply By: Phil P - Wednesday, Mar 05, 2003 at 18:45

Wednesday, Mar 05, 2003 at 18:45
Thanks for the replies - got the answers I needed !
Phil P
AnswerID: 14516

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