On 29/09/2017 01:17, T i m wrote:
On Thu, 28 Sep 2017 21:28:37 +0100, newshound
wrote:

snip

With it stripped, I have access to each battery terminal. Wikipedia
suggests that the safe maximum cell voltage is 4.2V, is that correct?

I believe it is (for Li-Po) but could be considered as an absolute
maximum (like a speed limit). If you charge to a lower and discharge
to a higher-than maximum voltage(s) you can both minimise the risk to
(and from) ... and increase the life (cycles) of such batteries.

After a little bit of charging, the cells are currently showing 3.72,
3.73, and 3.71 volts.

Standard procedure to see if you are likely to be at a risk from
potentially faulty / neglected cells is to charge them all to a safe
voltage (say 4vpc), leave them for a week and see how much they have
(self) discharged by. If it's not a lot and equal across all the cells
then the chances they are ok.

Thanks, very helpful tip. I've got enough DVMs lying around to be able
to build a charge/discharge setup measuring the voltage of each cell.
Might even get some of these

http://www.ebay.co.uk/itm/New-Mini-D...19.m1438.l2649


There's a little "floating" circuit board which,
to judge by the layout and wire colours, takes the charging current as
input, and has charging wires going to each end of the stack with sensor
wires to the intermediate junctions. This must be the "safety" circuit,

They are generally referred to as the / a 'Battery Management System'.
They both monitor the charge and 'hold back any cells getting close to
the maximum voltage to ensure they don't become overcharged whilst
also both disconnecting the load once any cell has reached the 'low'
voltage threshold and might (also as mentioned elsewhere) 'protect'
the battery from being charged at all, once it goes below a very low
voltage threshold (for the reasons stated above [1]).

tomorrow I will check whether that has gone open circuit.

I'm not sure how you will do that without a variable PSU plus your DMM
etc?

That's exactly how I'll do it.


I can live without the other features of the unit, if the safety circuit
looks OK I might just rewire it direct to the charging socket, and dump
the control board which does all the other stuff. (I don't normally use
it for jump starting, I use it to power CCTV).

Many people (mostly RC / aero moddlers) go without the BMS becuse:

It would have to manage very high currents.
It would add weight.
They use a low voltage alarm that (ideally) monitors each cell.
They use a sophisticated charger with a 'balance lead' / function for
bench charging.

Also as mentioned elsewhere, be areful with them as they can go off
like a firework (check Youtube for 'Li-po fires' and see howm may
'fire protection' charging bags and boxes you can get for Lithium
batteries and what levels many manufacturers of commercoial / domestic
packs go to to minise the risks (non resettable fuses to prevent
re-charging over discharged batteries and mutiple temperature sensors
etc).

Oh I will, I am fairly used to being around dangerous stuff. High
voltages or currents, various nasty chemicals. Radioactive sources. Not
so much experience with pyrotechnics, but in truth for an uncontained
laptop battery the risk is temperature and smoke, as long as it is away
from people and anything else inflammable.


Cheers, T i m

[1] I believe the risks from internal self destruction are because
large sharp crystals can grow under certain circumstances and punture
the internal insulators (or somesuch).


If it does look like it is recoverable, I will give it a few cycles in a
safe environment, and relegate it to jump starting duty.