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.
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
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!
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
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
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.
[email address is not usable -- followup in the newsgroup]
On Sun, 10 Nov 2013 21:07:28 +0000 (UTC), firstname.lastname@example.org
(Andrew Gabriel) wrote:
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.
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.
On 10/11/2013 20:38, email@example.com wrote:
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.
On 10/11/13 20:38, firstname.lastname@example.org wrote:
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
(in-ep-toc’-ra-cy) – a system of government where the least capable to
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