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

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.

Is it possible that I use the said charger for the said batteries? If so, what are the steps to modify the charger to suit the batteries?

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

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.

• The first line was a bit sharp, but I liked it - it made your answer more memorable. Another point: the manufacturer says 0.4A, you say 0.1A. Which maximum current should I use? – Vorac Jun 14 '12 at 8:39
• Mine, of course! ;-). The 100 mA is a rule of thumb for long time charging. "Initial charge" refers to the higher current when the battery's voltage is really low. That will decrease as the battery's voltage rises. – stevenvh Jun 14 '12 at 8:43
• ugh, from what i've learned about lead acid batteries, if battery is totaly flat (voltage below 12V in general, but it depends on amount of cells and properties of those), you should charge it with very little current untill it reaches nominal voltage (above 12V in general) and then you boost it with 1/10 C or more (again, depending on properties of cells). If battery specs say 0.39A i would assume, it's safe to feed it this current. At least if it's not totally flat. batteryuniversity.com/learn/article/… – miceuz Jun 14 '12 at 9:22
• @miceuz - You could be right. By "really low" I didn't mean completely depleted. I've never used lead acid under those conditions. – stevenvh Jun 14 '12 at 9:24
• @steventh yeah, and by initial charge they could mean high current bulk charge :) – miceuz Jun 14 '12 at 9:27

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.

• The light bulb idea is cool. However, it it seems it will take a load of those, connected in parallel. 12V / 0.4A = 30 Ohm ; 220V * 220V / 30 Ohm = 1613 W – Vorac Jun 14 '12 at 6:31
• @Vorac: I thought of connecting the light bulb in series to the charger. A single bulb with a 1W rating will limit the current to 80mA if 13V are applied. However, both the resistance of the light bulb and also the internal resistance of the transformer are non-linear, so you can use these type of calculations only for a rough estimate. – 0x6d64 Jun 14 '12 at 11:12

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.