Is it possible to find out how much charge remains in a drill/driver battery? Connecting a multi meter doesn't seem to give sensible figures. Any way of measuring the charge remaining?
Also, I have a rechargeable torch with apparently a sealed lead/acid battery. Can I simply charge this whenever, or will the battery develop a memory? Should I wait till it goes flat?
Lastly, I have an SIP 18v circular saw about 9 months old with 2 x 1.9 a/hr batteries, but they don't seem to last any length of time at all - never have done. Anything I can do with them?
Generally they need to be tested under load, so that with a given load, for a given amount of time, the voltage and/or current will drop by a certain amount, and from that it ought to be possible to calculate the charge left. Clever battery chargers do the complete opposite, but I'm not sure if there are any clever battery monitors that will simply tell you if the battery is any good.
If it is lead-acid, then *don't let it go flat*. Keep it fully charged at all times, and you will get a good many years life out of it. The memory for lead-acid tends to be the fact that they won't forgive you if you let them go flat - they have a memory better than an elephant (something to do with permanent and irreversible chemical changes that only happen to a flat battery).
At 18v, 1.9x2 AH, which we will call 4AH to be generous, would deliver 18x4 = 72W for a full hour. That is being very generous. I'm just guessing here, but assuming the saw sucks up 200W of power (reasonable?) then we are talking about a maximum time of 72/200 = 1/3 of an hour, or 20 minutes. Add in the 'real world' factors of differing loads, loss of voltage over the discharge cycle, duff batteries or inaccurate labelling, better values for my guesses, heat losses, and you probably would not get much change out of ten minutes.
I'm assuming these are two 18V batteries in parallel, or used one after the other. If they are two 9V batteries used in series, then halve that theoretical 20 minutes down to 10 minutes straight away.
Not with much certainty. The only realistic way is to charge it with a decent delta peak sensing charger. You then get an indication of how flat it was by the charge time.
Sealed Lead Acid batteries don't have a memory effect and quite like being "float charged". The thing they like least is deep discharge.
Re-cell them with better quality (and higher capacity) cells. It is surprising the improvement you can get from some tools just by replacing the supplied batteries.
General tips for NiCd / NiMh batts:
Discharge them until they shown signs of fading - don't carry on using the battery after that point.
Use a decent charger / Don't overcharge
Don't charge a hot battery - if you have been working it hard, then let it cool for half an hour before charging. (helps to have three batts for this reason)
Periodically recharge them if left in storage for any length of time.
The voltage of NiCd or NiMh cells is almost constant from full charge to flat, so it can't be used to judge capacity accurately. The only way to test capacity is to use a device that charges the battery then discharges it at a fixed current and monitors how long it lasts until the voltage falls.
Lead acid should be kept topped up as much as possible. If you let it run flat the plates sulphate and it loses it's ability to take charge.
Standard battery packs are often only rated between 1.2Ah or 2Ah. That's not an awful lot for a very heavy load. Maybe you're underestimating just how much cutting you do get with them. Also note that the more you force the saw the higher the battery demand will be as the motor struggles against the force. Go easy on the cutting and let the blade keep a relatively high speed.
I'll also remind everyone that as soon as your tool battery starts to go "weak" recharge it immediately. As tempting as it is to finish drilling the hole, it is damaging the battery because the first cell to go flat gets run in reverse by the other cells in series and may lose capacity leading to it going flat even sooner next time.....
Contrary to popular belief you don't need to discharge NiCd battery packs completely before you charge them. You can top them up at any time and just give them a full discharge every so often to keep them fit.
And those dumb "7 hour" chargers where you are advised not to leave the battery on charge? Dump them and get yourself a proper cordless power tool!
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