Using car charger for small sealed-type lead-acid batteries

As many members here have explained in depth about the charging process I’m not going to re explain it again. But to say in simple terms don’t use the charger you have. It will destroy the batteries in no time. I have had made this mistake in the past. It’s a matter of time before the lead plates breakdown and settle to bottom on battery case causing a short to occur. If you see the leaflet that’s comes with the small lead acid battery you will see the manufacturers say that you should charge at C/10 regularly and for an occasional fast charge C/4.

In your case C/10 would be (1000mAh/10=100mA charging current) is the recommended one. Btw 1Ah=1000mAh.

If you want some good life out of it you might want to stick with this rate. Get a 12V charger with this rating or even a cheap bench top power supply (you can easily find on eBay) can get the job done. But I think you will have a hard time finding a charger below 500mA or 1000mA at 12V.

I want to charge a couple of small (1Ah 12V) sealed-type lead-acid batteries. I have a Bosh KL 1204 car battery charger. The charger's nominal current is fixed at 2.3A, while on my batteries it is said "Initial charge current <= 0.39A.

As your charger was designed for a car 12V battery, it has same voltage parameters for your tiny 12V battery.

So most probably YES you can charge using this charger if and only if you limit the charging current to the battery.

You will need to adapt & check a few steps:

1. Limit inrush current to I< 0.39A. If you use a 12V x 5W lamp as current limit resistor, it has the advantage of being a non-linear resistor and maximum current will be even better controlled than using a conventional (Ohmic) power resistor.
2. Check if “final” battery voltage is < 14.4V. This is important, as your battery charger would be expecting a much larger battery and there is a non-negligible possibility that it’s “idle current” would not be voltage-limited to 14.4V. If it goes above 14.4V, then do NOT use your charger.
3. Check if charging voltage is reduced after a given time at V </= 14.4V. Check and annotate voltages (and currents) periodically along first charging cycle, then estimate charged accumulated energy (V x I x time_interval). Most probably, if your charger detects a lower current to charge up to absorption voltage of 14.4V, it would terminate this absorption mode/stage and enter a floating mode with reduced final voltage of 13.8V, a safer voltage to leave your small battery connected and unattended. If not reaching this lower voltage, then you now know that you should not leave the battery being connected to the charger for longer than the time necessary to archive 40% more than your rated capacity; in your case, 1.4AH.

I have used a simple method for a number of years, Connect a 2.2 Watt (12Volt) dash board type lamp in series with the battery on a 12 Volt charger, The light limits the current and indicates what is going on.

The easiest thing would probably be to use something like a LM317 because of it being cheap and its quite low maximum current. If you can find an internally limited regulator limited at a lower current, that would be even better.

$$ mathrm{2.3 A > 0.39 A} $$

End of story.

Whereas other rechargeable batteries most often use a fixed current, lead cell batteries are charged with a fixed voltage, combined with a current limiter. Usually current limit is 1/10 of the battery's capacity (/h), so a 1 Ah battery is charged at 100 mA maximum. You can use the charger if you place a 100 mA current limiter in series with it.

Is it possible that I use the said charger for the said batteries?

No, the car chargers current will likely damage your battery.

If so, what are the steps to modify the charger to suit the batteries?

In theory, a resistor or incandescent light bulb in series to the battery could work. I however suggest charging the battery by connecting it to a constant voltage (13.8 volts for a 12 V battery). A bench power supply then can limit the charging current to 0.4A.

This method is obviously simple but comes with a price: It is slower than other methods and can decrease the batteries capacity over time. But I really like it because you don't have to worry about overcharging the battery.