# Determine battery voltage

I have removed the battery from an old portable dust cleaner. The acid has oozed out, and that is the reason the device does not work anymore. I want to replace the batteries, but their voltage is not indicated.

The only hint I have is the charger output. Is there a univocal way to determine which kind of batteries is meant to recharge (like a phone's 5V charger feeds 3.7V Li-ion batteries)? In that case, the charger is 3.9V and 0.5 VA (better, W).

Edit: Added photo. It was a pack of 2 batteries like the one showing in the picture, with my hand holding it for size comparison. Size is similar to a C battery, but not equal (approximately 4.5 cm height x 2.3 cm diameter - 1.77"x0.91")

• Post a photo of the batteries. A charger isn't a simple power supply, its voltage and current output react to what it thinks the cells are doing. A charger for any voltage of any chemistry might reasonably put out 3.9v when there's no working batteyr connected. Commented Jul 6, 2018 at 13:49
• Done! It was a pack of two cylindrical batteries like that one, with a black plastic ring at the bottom. Added size. Commented Jul 6, 2018 at 14:24
• I reckon that's NiMH, 1.2v per cell. Commented Jul 6, 2018 at 14:32
• So should any 2.4V setup do? Commented Jul 6, 2018 at 15:41
• 2.4v will do. I'm not sure how many alternatives you are considering as falling under 'any'. A dust cleaner may well take a lot of current, and a large cell like that would be able to deliver it. My cordless drill uses that size, and pulls more than 20A. So you may need a high current supply, which will need thick wires to it. Once you've measured the current that you need, you may be able to find a big enough buck converter on fleaBay or elsewhere that will fit into the space of the two cells. Then it needs only short low voltage wires, and use long thin ones from your 19v laptop supply. Commented Jul 6, 2018 at 16:34

Since you say this device is "old", the batteries were probably NiCd or NiMH. NiCd will be hard to find, for good reason. You could take a shot at replacing with NiMH, which have similar forward voltage and charge profile as NiCd anyway.

Fortunately, NiMH don't tend to blow up, like lithium batteries, when abused. They are more likely to ooze some corrosive goo.

• What I meant was their voltage, so that I can replace them with a power source fit for my device. Basically, I want to know how many V it requires, to buy suitable batteries. Commented Jul 6, 2018 at 14:06
• @JeffreyLebowski NiCd and NiMH cells both run at 1.2V fully charged. Commented Oct 9, 2020 at 11:11

Since you aren't sure of the old battery specs, you should test the device with a variable DC power supply to determine a reasonable range of working voltages and currents. Then you can start looking for battery replacements.

• That's not really necessary, it's obvious from inspection that this is a sub-C NiCd or NiMH, the only question is which, and your load test would not distinguish them. Commented Sep 8, 2020 at 14:00

You need to replace it with a battery of the same chemistry not just a similar voltage, the charging requirements of lithium cells is completely different to NiCd and NiMh cells for example. Get this wrong and you will have exploding batteries.

As Olin lathrop mentioned you could assume a NiCd or NiMh cell. Bear in mind that although NiMh cells have higher capacity than NiCd cells, they have lower current capabilities and you will see significant capacity derating if the current demands are high. Which could see you experience lower capacity in use with a NiMh cell even if it has a higher rated capacity than a similar NiCd cell.