[MRCA] Batteries; (Was On Patrol Cuttyhunk Island)

[Peter Gottlieb]

When I can get cells with straps welded on that's the best but in my case I got 
the cells out of surplus packs so what I did was use a Dremel cutting disk to 
carefully cut the nickel alloy weld straps leaving short bits I could easily 
solder to.  Then I just soldered short pieces of teflon wire to make up the 
pack.  These aren't protected cells, they are just standard A123 iron phosphate 
nanoparticle cells, in this case the 26mm diameter ones.  For the 515 pack it is 
a rectangular block of 8 by 2, and wired as 8S2P. For the 2000 I made a little 
jig to glue sets of 3 cells together then glued four of those to make 4S3P 
blocks.  These particular cells are rated 2.3 AH although they all do better 
than that with most over 2.4 AH.

I use BMS boards I but off of ebay which are all Chinese.  There are no great 
ones out there that are small and do exactly what I want. But many are close 
enough.  I want two things, cell balancing and under and over voltage cutoff.

These boards resistively load cells (or more precisely cell groups) which go 
over a certain voltage.  This preferentially charges the other groups.  It's a 
subtle loading but that's all that's needed because it improves the pack balance 
at every charge.  If you don't overcharge or overdischarge a pack it is amazing 
how long you can use it without any balancing or protection, as in years of 
daily use.  But you really should have a balancer especially with cheap Chinese 
cells if you use them.

These little boards have voltage monitoring for each cell group and will trigger 
on any one going under or over voltage by opening some power MOSFETs to 
interrupt charging or discharging as appropriate.

Without protection, if you under voltage a pack, immediately recharge it as 
leaving it over discharged can permanently damage it.

If you overvoltage an iron phosphate pack enough, cells could vent. It will be 
an odd but not overpowering kind of smell and the cells that vented will soon 
dry out and be useless.  Sort of like what happens to a vented electrolytic 
capacitor.  Any vented cells must be replaced, ones that didn't vent should 
still be fine.  If you over charge a cobalt oxide or similar lithium ion cell 
you can have a vicious fire or explosion.  Charging any kind of pack of those 
type of cells is inherently risky, more so with amateur equipment which hasn't 
gone through a lot of safety qualification.  I have just a few of that type of 
pack around and I charge on a metal sheet and never overnight.  In contrast, 
I've always charged iron phosphate cells without worry although I try not to 
ruin them with overvoltage as sometimes I've put a lot of time and effort into 
building the packs.

With the BMS board you just treat the pack like a lead acid battery.  A 12 volt 
pack?  I charge to 14.4 volts float.  28.8 volts for the 515 pack.  You can 
actually go higher but it hurts you on charge as the BMS will cut out on 
overvoltage before the cells have fully absorbed full charge and you then have 
this cycling of too much current then none.  Yes it will work but not ideally.

Maybe tomorrow I'll play some more with the solar panels.

Peter




On 9/24/2016 3:48 PM, Al Klase wrote:
> Hey Peter,
>
> I'd like to hear more about your batteries.
>
>   * Are you using holders or welded straps?
>   * Are they "protected cells"?
>   * How do you control charge and prevent over discharge?
>
>
> Inquiring mind want to know.
>
> Al
> Al Klase – N3FRQ
> Jersey City, NJ
> http://www.skywaves.ar88.net/
>
> On 9/24/2016 12:03 AM, Peter Gottlieb wrote:
>> Great fun to get out in the field for some portable operations with mil 
>> radios.  I was out with a digital head PRC-515 as well as a Racal Syncal 
>> 2000.  The 515 has a H-189 handset, the racal a Clansman RA-250.  Both have 
>> lithium ion packs I built out of A123 26650 M1B cells:  4.6 AH 26.4V for the 
>> 515 and 6.9 AH 13.2V for the 2000.  They powered the radios flawlessly and 
>> were very light.
>>
>
>
>
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