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

Sun Sep 25 21:30:45 EDT 2016

Make sure to get a BMS which is designed for the cell chemistry your cells use.  
Different ones have different voltages.

Peter

On 9/25/2016 8:55 PM, Al Klase wrote:
> Pater,
>
> Ah ha!  The secret word was "BMS."  I'd been kultzing around with 18650 cells, 
> but was uncomfortable with keeping them under control. Thanks for the info.
>
> Al
>
> Al Klase – N3FRQ
> Jersey City, NJ
> http://www.skywaves.ar88.net/
>
> On 9/24/2016 5:47 PM, Peter Gottlieb wrote:
>> 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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