Testing power tool batteries

I know this has been discussed before, but is there really no way to test a power tool battery?

The 3 x 12v & 2 x 14.4v batteries on my Makita stuff are starting to play up a bit, but I'd like to know the full SP on each battery.

I have a multi meter, that's about it.

Reply to
The Medway Handyman
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As a rough comparison between packs you could stick a car 21W brake lamp bulb or 20W low-voltage (e.g. downlighter) bulb across the battery terminals. 20 or 21 Watts @ 12 Volts is around 2 Amps so a 2Ah pack should run the bulb for about an hour. A 50W downlighter or 65W headlamp bulb would flatten it in 15-20 minutes. All figures *very* approximate, but ball-park.

Reply to
John Stumbles

That is what I do to cycle batteries or test their capacity but I also hang a voltmeter across the terminals and keep an eye on it. You don't want to take the battery down below about 1v/cell or you run the risk of reverse polarising the weakest cell in the pack and naggering it.

A nominal 12v pack will have 10 cells in it so the minimum terminal voltage is 10v. For the 14.4v pack with 12 cells the minimum terminal voltage is 12v.

Reply to
Dave Liquorice

Does it matter what the 'numbers' are, though: surely all that matters is whether the batteries are actually performing OK in practice?

David

Reply to
Lobster

Connect to a car bulb - say a 21 watt stoplamp one with your meter in series set to current. Note the current in amps and time how long it takes to go noticeably dimmer. Convert the time to hours and parts of an hour - ie 1.6 hours etc. Multiply the two together and you get the amp/hour of the battery.

Reply to
Dave Plowman (News)

Not really - the capacity of cells is quoted at the 10 hour rate, if you discharge at a rate to last only one hour the capacity will generally be far less.

AWEM

Reply to
Andrew Mawson

The rating's a bit of a waste of time then, since the 'normal' consumption of the tool it powers will be far greater than the '10 hour rate' current.

Reply to
Roger Mills

Slightly OT - but it may be of interest to some people.

The batteries have died on one of my 12v drills - so I have removed the cells from the battery pack and connected a cable to the contacts, with a cigar lighter plug on the other end. This plugs into one of those portable power packs with which you're supposed to be able to start a car. So, although the drill is no longer 'cordless' it can still be used in places where there's no mains supply and, of course, the external power pack has a much greater capacity than the original internal one.

Reply to
Roger Mills

yes, so you don't get as much delivered capacity as on the battery label

illogical.

You need to test at least 2 things:

- capacity

- self discharge

The latter can be tested by fully charging, waiting 1 month then seeing how much it capacity it supplies a known load. Not the most practical time-wise, but if you have a battery you plan to retire if not upto it, its doable.

If you use a dead battery pack to add a 12v cord, you could then probably use those old 5A computer NiMh batt packs.

NT

Reply to
NT

What are you saying is illogical - the rating, or my statement that it's a waste of time?

Reply to
Roger Mills

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