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Fixit



Joined: 22 Nov 2012
Posts: 161
Fixit
6 VOLT 4.5AMP HOUR RECHARGEABLE SLA 4.5AH SEALED BATTERY

If I use a 500 Mah charger to charge this battery, is there a formula to determine the charge time based on the starting voltage ?

Thanks gentlemen and scholars.

Andy
Post 01 May 2014, 03:29
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17279
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revolution
Using the voltage is tricky because it depends upon a few factors unrelated to the charge level. You might be able to get an approximation but there could be a considerable amount of error.
Post 01 May 2014, 03:54
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neville



Joined: 13 Jul 2008
Posts: 507
Location: New Zealand
neville
The open-circuit voltage isn't very useful, but the on-load voltage with a load of up to "0.1C" (0.45 Amps or 450mA in your case) can give you a rough idea of initial state of charge.

If the voltage falls below 5.5V the battery is exhausted and should receive 4.5 Ah to fully recharge.

Your charger is presumably rated at 500 mA (0.5A) continuous current output. Chargers are not rated in mAh or Ah of charge.

So if it supplied constant current fort the entire charge (which it shouldn't because that's not good for SLA's) it would require 4.5Ah / 0.5A = 9 hrs to fully charge. In fact the current will reduce over time and the charge time will be longer than 9 hours, maybe over 10 hours. The charge should also be stopped when the voltage reaches about 7.4V

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Post 01 May 2014, 05:27
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Fixit



Joined: 22 Nov 2012
Posts: 161
Fixit
Thanks for the good info.
Post 01 May 2014, 19:04
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gens



Joined: 18 Feb 2013
Posts: 161
gens
http://www.batteryweb.com/manuals/techman.pdf
might help

wiki has some simpler graphs (on the right, theres also a link to a site that i didn't check[11])
http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery#Measuring_the_charge_level

didn't find a formula, but you can try extrapolate
remember that the voltage is higher if you just charged it, since it didn't settle in the chemical stuff
also that accumulators loose capacity over time (over charge/discharge cycles mostly)
Post 01 May 2014, 20:13
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cod3b453



Joined: 25 Aug 2004
Posts: 619
cod3b453
A rough formula in terms of energy transfer would be Time=Capacity/(Current * Efficiency) or 4500mAh/(500mA * 80%)=11hrs for a full charge. Then, taking the normal 12V battery numbers, a 6V battery should be roughly 100% at 6.25V, 50% at 6V, 10% at 5.75V and 0% at 5.25V. You can then interpolate the state of the cell from these numbers to work out how long it should take - note this is an "s" shape curve so not such a straightforward calculation.

In reality this varies with temperature, the charge current, the state of the cell and its plates (related to age as well), the construction/chemistry and the type of charger. Ideally you'd have a 3 stage system that performs: constant current("bulk")->constant voltage("absorption")->constant voltage("float") to achieve maximum capacity charge without overcharge.

As you've got just the bulk stage, you'll want to switch off at the 100% mark to prevent overcharge but the actual capacity will only be ~85% or 3800mAh so the adjusted full charge time would then be about 10hrs.
Post 02 May 2014, 17:50
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