Charging Li Ion batterry from DC source

[news]

I have a power tool (chainsaw) which is used on site at night where noise is a problem. It uses a lithium ion battery of 36V 3Ah which lasts about 20 minutes of use. The 330W 240v charger takes about 1/2 hour. I have 3 batteries which is fine many nights but could do with charging them from a DC power source like a series of lead acid leisure batteries, would this be feasible? Starting a generator defeats the object and an inverter off a vehicle battery ( we have welfare units with isolated 12V battery) would need 27 Amps even if it were loss less.

AJH

[Bill Wright]

Would it be feasible to power the saw from three 12V 15Ah deep discharge batteries, via a short cable? Obviously if you are working up a tree the answer would be no. I had a problem a few years ago where we had to work in some long and rather labyrinthine unlit voids (think sewer tunnels but without the sewage; can't saw more than that). We only realised the problem once we were on site (had been told that the lighting worked) so we used caplamps for a day, then I rigged up an array of 12V 10W halogens on a heavy photographic tripod and powered them from a 30Ah deep discharge battery. I had a good long cable so it was only necessary to move the battery (which was on a trolley with the tools and equipment) every now and then. Incidentally, years ago a communal TV system installer of my acquaintance used to tap into the 55VAC line power carried on the distribution trunks to feed a 24V headlamp bulb (the connection being to the non-common terminals, so the two filaments were in series.) Frankly I thought it was more trouble than it was worth!

Bill

[Andrew Gabriel]

It's not easy. You need a supply which has a very precisely set voltage and current limit, the value of which depend on the particular Li Ion batterry. If you are fast charging them, it's also essential to monitor the temperature. Also you must take them off charge when the charge is completed.

The really nasty part is that if you get this a little bit wrong, the battery is liable to explode into flames like a firework (and like a firework, you can't put it out with a fire extinguisher, although that might be useful to put out whatever else it ignites). There are videos on youtube.

So unless you have a good quality electronics bench supply with accurate voltage setting (to 0.1V) and current limiting, it's not safe to do so.

I have done it with a mobile phone battery - doing it with something bigger has bigger risks.

[Toby]

Inverters are pretty efficient, somewhere around 85% IIRC, so this is probably going to be the easiest & safest way to do it, what is the problem with taking 27+ amps from a leisure battery, the total drain on the battery will be roughly the same with a slower charger anyway?

As I understand it, when you charge a Li Ion, it is charged at high current for the first half of the charge cycle, then the current drops off, so your 330w charger probably won't be drawing the full 330w from start to finish anyway.

Granted, the efficiency will be better with a DC/DC solution, but I don't expect it will make an appreciable difference, especially if you can recharge the leisure battery during the following day anyway.

[Archibald]

Three fully charged lead acid car batteries? Probably cheaper than the lithiums but the downside is the need for a cable and some means of attaching it to the tool.

A H&S problem I would assume if you trail a cable to such a tool?

How long can a chainsaw be used for incidentally, a Duracell PP3 might have enough juice to send some hi vis spotters into white finger mode.

AB

[The Natural Philosopher]

You can get model (aircraft/boat/car) charger which will do thus BUT they are designed to hook onto the raw battery itself, and not expecting any protection shit on the way.

You might do better with a 12v inverter

[F Murtz]

If the charger is supplied from a power pack (wallwart in American)which supplies 12v dc to the charger then it should be easy to use the proper charger

[Andy Burns]

Might just do better with a few more batteries ...

[Martin Brown]

They are as Boeing will testify seriously inclined to self immolation.

Since he needs under 500W to power the charger it might be easier to get a mains inverter and run the existing charger from that (taking good care with mains voltages in the field). There will be an efficiency hit but probably only 10% or so. Assuming here that the charger is sufficiently well designed to cope with a ropey inverter waveform. I expect the charger will kill a nominal 300W toy inverter.

Otherwise you would need commission a charger from someone who is competent to design it with the appropriate temperature, time, current and voltage safety interlocks. Get it wrong and you brick the battery or even worse start a lithium metal fire.