Husqvarna BLi150 36V Li-Ion Battery Charging—Charger Intelligence & Safety Features

I have a Husqvarna garden machine with a 36v 4.2Ah battery like this https://spaldings.co.uk/products/sp-website/g...y-powered/husqvarna-bli150-li-ion-36v-battery

It’s a pretty standard sort of pack although of course there is no such thing as a standard mechanical and electrical interface on these as every manufacturer does its own thing. I have lost the charger which is this
https://www.husqvarna.com/uk/parts-accessories/battery/charger-qc80/967335651/ 
There are 4 large connections plus 4 tiny pins that are unconnected. I connected the red and black battery terminals to a bench power supply and it charges fine. It briefly takes at least the 4.5A that my supply will deliver. I charged it to ¾ at 2 A though to be safe.

Questions:

What is in these chargers? Appears to me that, unlike the type you get for NiCd or NiMH electric drill batteries, there may be no intelligence in the charger to shut off the charging when a certain condition is reached or to detect battery temperature. Instead it seems to be just be a DC supply as the battery pack itself has a big PCB with cell balancing and LEDs to show charge and error states.

If it’s that simple am inclining to carry on using the bench supply and save PLN50 on a new charger as I rarely use the battery. Is there any safety risk to me in going this?

What do the other 2 terminals do? They only have thin wires internally so presumably are sense wires for the machine, which does connect to them. The charging does not seem to need them as it charges without them connected. Unless it is a pack voltage sensing signal to stop the charge.

It’s a notional 36V battery with 20 standard Li CR18650 cells in a 2x10 arrangement. I calculate that as 37V at nominal 3.7V cell voltage and 42V at 4.2V. What voltage should I set the bench supply to? 42V may be the safest option just in case it does not auto shut off.

Would the official charger be say limited to 42V or will it be a little higher and rely on the battery to auto shut off?


If I connected to an unlimited current source would the battery limit the current? Based on its quoted charging time the official charger seems to give an average of 1.8A up to 80% charge level. 

Li-ion charging can be forgiving (provided you don't exceed voltage and current ratings), but don't expect the batteries to last long if you don't do it right. The charger I have for a  high voltage power tool (a line trimmer) employs a standard charging profile, and sensse the temperature of the battery pack. This alows max charge rate while prevent the temperature  getting too high. See this article at battery university for some useful guidlines, and a few more herecheers,Richard

Thanks.

On reflection I see that the 2 terminals I am connecting too are direct
connected to each end of the stack of cells. Thus there is no way the PCB in
the pack can cut off the charge when the cells are full, nor can it limit the current.
However it should cell balance and correctly indicate the charge state. I suspect
that the 2 “spare” connections are there to tell the charger that it is full so
the charger can cut off. They can’t be for the main charge current as the wires are
too thin to take even 2 amps without some significant volt drop.

 The alternative is that the charger is a
fixed 42V (or whatever the cells need, and maybe slightly variable depending on ambient temperature) at up to 2A. Then when cells are full they naturally stop
drawing current without any disconnection needed by say a FET.

Based on your links Richard I guess one possible
reason for the 2 thin wires is to tell the machine that the battery voltage is
too low for the machine to safely carry on draining the battery. The machine could
do this by measuring the battery voltage on the main terminals but the balancer
board probably has a more nuanced view, e.g. if one cell is getting a low lower
that the rest.

Richard's reference Battery University is a good one.I've charged many Li-ion battery packs (up to 14 cells in series) using a bench supply with the voltage limit set to the desired fully-charged voltage and the current limit set to the desired charging current (usually the recommended charging current per cell times the number of cells in parallel).  The fully charged voltage may be determined by the cell manufacturer's datasheet, usually something like 4.1 or 4.2 V/cell, times the number of cells in series.  Setting this voltage a little lower (like 4.1 or 4.0 V/cell for 4.2 V cells can significantly  extend the cycle life of the pack at the possibly-acceptable expense of some loss of run-time of course). But charging should not be allowed to continue indefinitely.  As the battery charges the current limit of the power supply (PS) defines the Constant Current (CC) portion of the charge cycle and the voltage rises.  Once the voltage reaches the voltage limit setting of the PS, the Constant Voltage (CV) portion of the cycle is entered and the current tapers off. The data sheet usually recommends a cutoff current (multiply by the number of cells in parallel).  Once the current tapers to below that threshold, charging should be terminated.  
An added protection is often to terminate charging after some amount of time has elapsed, usually a couple of hours longer than the expected time required to charge the pack.  Maybe put the PS on a timer?It is important to not charge the pack when too cold (again specified by the data sheet, something like below 0 C) to avoid damaging the cells through plating of Lithium.  And charging when the cells are too hot should also be disallowed (see datasheet, in my case above 45 C if I remember correctly).
One thing to be careful of when charging with a bench supply is that the supply can handle having a voltage applied to its output when the supply is powered off or its voltage is set below the voltage of the battery.  It could be damaged by current that flows back into the supply output if it is not designed for that.  To address this, I often connected a diode in series with the output of the supply to avoid back-feeding of current into its output.  This complicates the voltage setting for the CV portion as you have to compensate for the forward voltage drop of the diode, which decreases as current tapers, so you may have to experiment a little to get this setting just right.I hope this helps.  Be careful.  Be safe.  -JD