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Old 04 February 2007, 07:59   #1
RJH
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Battery question

I have a flat(ish) 85amp marine battery. How long should it take to charge it back up with 4amp charger. Probably a very simple answer but electrics not one of my strong points. I've had it on for several sessions of a few hours each but still not enough to start the engine so wondering if the battery is knackered or am simply not charging for long enough. All thoughts gratefully received. Levels checked and all ok.
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Old 04 February 2007, 08:08   #2
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85 4 plus a bit for losses...about 25+hrs.
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Old 04 February 2007, 08:23   #3
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Thanks JW. I was afraid it was going to be something quite complicated like that Like I say, electronics all seems a bit like the dark arts to me.

Much appreciated.
RJH
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Old 04 February 2007, 09:45   #4
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You don't mention what type of charger you have. In a standard car charger, as the battery voltage rises the charger regulates the current which decreases from 4amps to much less by the time the battery is well charged so it's more of a curve than a step function (current vs time). So charger will probably only output 4amps when battery is quite flat (most chargers have an ammeter so you can read the current).

If you have an intelligent 3 or 4 stage charger then that's a bit different as these will deliver full current (4A in your case) until the battery is approx 80% charged and then reduce current for remaining charge steps. These will charge battery a lot faster than a standard charger of equivalent amperage.
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Old 04 February 2007, 11:43   #5
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I'm pretty sure its a standard car charger so will work on that basis. Thanks.
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Old 04 February 2007, 14:47   #6
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In that case I'd guess it will take a bit longer to charge fully (depends on how flat it is). Average current may be something like 2-3 Amps rather than 4 (over duration of charge cycle).

You mention it's a Marine battery so I'm guessing it's a deep cycle. If it's lead acid they really don't like being flat for any period of time (not sure about the gell type) so it may not reach it's original capacity when charging. Batteries can be complex beasts.

Hope this helps!

Al
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Old 04 February 2007, 14:59   #7
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Was flat for around a month so it might be that its coming to the end of its useful life. Will give it a go to see if it recovers and if not then replace it.
Thanks for the tip.
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Old 04 February 2007, 15:29   #8
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Quote:
Originally Posted by RJH View Post
Was flat for around a month so it might be that its coming to the end of its useful life. Will give it a go to see if it recovers and if not then replace it.
Thanks for the tip.
As already mentioned modern batteries often get the hump if left flat. You may find it charges up but won't hold its charge, or has lost a lot of its reserve capacity. I'd be leaning towards changing it as a precaution...
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Old 04 February 2007, 16:38   #9
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How di do RHJ,

not that simple I am afaid (as you guessd) but for most part, you do not need to know the rest.
The charging of any Battery and Capacitor is based on a timing constant
T(time) constant of charge T(c) = Capacitance x Resistance or T(c) = CxR where R = Resistance of circuit + dynamic resistance of battery (r)
Thus T=C(farads) x (R+r) Ohms answer in seconds.

This is to the best of my ability from years ago.

You nev mentioned teh condition of the battery, this is of huge importance.
Plates get build up of crap and bend which can short out cells etc.

If two cells short plates due to build up of Electrolytic deposit or indeed with Heat they buckle and come in contact, the battery will appear to charge up very quickly but it will have a lower voltage and heat up in the proess but it is useless at this point. The Internal resistance can only be measured using Shunt meter which I doubt you will have and the voltage meter is useless unless really shagged with the plates in full contact where it will not read 12+ volts (up to 13 in charge mode). PS unless using high Impediance (Z) mode of meter, you will have very little of it left by now!!!!!

If a Battery (lead acid) has been left go flat, it will not recover from this fully and its life is seriously shortened.

There are a few batteries that use a GEL Acid type and do not spill. They are high TORQUE which really means high heat tolerance as if you depleate them quickly the heat up hotter than when charge (given you are drawing up to 80 Amps min with Starter motor. Finally and sory for the info but a battery that says 80 AMPere hours as in AMPx Hours does not actually mean 80 amps for one hour or 1 amp for 80 hours, that is more of a theory. Again the Internal Resistance and Heat of the battery will decide this more as in condition of battery and ambient conditions of use TEMP etc.

I will have to check on previous threads as to which one were recommended.

Again ask Local Mechanic on RNLI lifeboat.

There is much written on this and there are many who know more than I do.
Get the battery checked each season in Garage having charged for 24 hours,,,
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Old 04 February 2007, 17:24   #10
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Thanks guys. Wow, I've just remembered why I didn't enjoy physics at school! I always thought batteries were pretty simple but just shows what I know! Will take a lot more care in future.

Rgds
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