Maker Pro
Maker Pro

Can I charge 12v 8.5ah lead acid with 12v 9A charger?



Jan 1, 1970
I charge it now with a 12v 1.2A in 14 hours.

If not could someone explain why? Is it the heat build up?

John Popelish

Jan 1, 1970
Ottawawest said:
I charge it now with a 12v 1.2A in 14 hours.

If not could someone explain why? Is it the heat build up?

A 9 amp charger can supply 9 amperes without overheating
(supposedly). If your battery draws less than this, the charger
should just run very cool. However, if the charger has poorly
regulated voltage, and the voltage goes up as the current goes down,
then it may push too much current through your battery. Heat build up
is only one bad effect of charging a battery too fast.

But you can slow the charger down by adding a little resistance in
series. A 12 volt light bulb may work. Pick one that gives you the
current you desire.

Byron A Jeff

Jan 1, 1970
Ottawawest said:
Can I charge 12v 8.5ah lead acid with 12v 9A charger?

I charge it now with a 12v 1.2A in 14 hours.

Actually no again. At least not fully.
If not could someone explain why?

Because you cannot fully charge a 12V lead acid battery with 12V. In fact
you'll never get more than an 80% charge.

To charge a lead acid battery requires at minimum 2.3V per 2V cell. A 12V LA
has 6 cells and therefore requires at minimum 13.8V. This won't get you a full
charge, but is safe to charge at indefinitely.

To get a full charge actually requires 2.4V/cell. This is a voltage of 14.4V.
But you cannot charge indefinitely at this voltage.
Is it the heat build up?

Heat, gas buildup, water boiling dry are all factors. And the more charge you
drop into the battery, the faster it'll reach a state where damage occurs.

Most battery manufacturers suggest not charging at a rate that is faster than
the AH capacity over 3 (i.e. C/3). For your battery that would be a bit under
3A. A 5 ohm resistor in series would give you 2.88A with a small voltage drop.

You need to do 4 things:

1) Raise your charge voltage to a steady 14.4V
2) Put in the 5 ohm series resistor to limit current.
3) Monitor the current the battery is consuming. when it drops to C/50 (170 mA
in this instance) then the battery is fully charged.
Always trickle charge at 13.8V. This voltage is safe indefinitely.

Hope this helps,



Jan 1, 1970
here is how it works
If you had a 12v 1000 MAh battery that's a 1-Amp battery
if you charge it at 100ma , it would take 10 hours
if you charged it at 500ma it would take 2 hours
there is a mats relationship there .

Experts always say add a little more time due to losses etc

Charging any Battery with a very high current will of course cause a heat
build-up and eventually a shortage in battery life
if I had a 12v 8.5A battery I personally would charge it at about 14-15v and
about 400mah for around 24-hours

By adjusting the charge current and time you can fully charge
the battery in a few hours to a couple of days .

a slow charge is far better and kinder to a battery than
a high fast one .I suppose it depends on how many times a week you
charge it & how quickly you need the battery and if you like spending
money on replacement batteries every couple of years

hope the info helps

de Paul


Jan 1, 1970
Depends on the type of battery.
Is it a plate/wet cell or a dry sealed microcell?
If it's a dry with microcell and you try to charge it too fast it'll
The max amp rate should be on the battery somewhere.(I us US or
powersonic batteries and the max rate is usually 850milliamps.