Nimh in parrallel

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antiscab
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Posts: 2357
Joined: Mon, 26 Nov 2007, 05:39
Real Name: Matthew Lacey
Location: Perth, WA

Nimh in parrallel

Post by antiscab » Sat, 10 Aug 2019, 17:11

So I did a dumb thing, that I thought I should share

I came home to a hissing and spitting sounds coming from my home storage battery (2 x strings 102cells 30Ah nimh prismatic cells in parrallel)

The batteries were sitting at ~135v, and one string was super hot (and making bad noises). The other string was luke warm

My working theory:
Basically both strings charged to full, one got hotter than the rest. The voltage of the hotter string started to fall (its a nimh thing).
The hot string basically coped the full charge current.
Self discharge increases the hotter the it gets, eventually the hot string's self discharge rate is higher than the charge current and starts discharging the other string.

oops.
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

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jonescg
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Posts: 2813
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.

Re: Nimh in parrallel

Post by jonescg » Sat, 10 Aug 2019, 17:46

Well glad that didn't cause a fire.

But geez, NiMH is pretty ordinary eh? Would a fuse have saved it?
AEVA National Secretary, WA branch vice-chair

antiscab
Senior Member
Posts: 2357
Joined: Mon, 26 Nov 2007, 05:39
Real Name: Matthew Lacey
Location: Perth, WA

Re: Nimh in parrallel

Post by antiscab » Sat, 10 Aug 2019, 20:51

The cells and wiring are rated to 60A continuous. I don't think a fuse would have helped.

Voltage monitoring (stay below 145v, as the runaway condition would have been triggered by an excursion above 150v) or temp monitoring may have done the trick.

The former I'll implement.
I have heaps more cells, so can rebuild.
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

dRdoS7
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Joined: Fri, 28 Sep 2018, 12:28

Re: Nimh in parrallel

Post by dRdoS7 » Tue, 13 Aug 2019, 07:42

antiscab wrote:
Sat, 10 Aug 2019, 20:51
The cells and wiring are rated to 60A continuous. I don't think a fuse would have helped.

Voltage monitoring (stay below 145v, as the runaway condition would have been triggered by an excursion above 150v) or temp monitoring may have done the trick.

The former I'll implement.
I have heaps more cells, so can rebuild.
Voltage differential measurement?

Interesting, how much did the setup cost. How do you charge them? What do you run off them.

Thanks,

dRdoS7

antiscab
Senior Member
Posts: 2357
Joined: Mon, 26 Nov 2007, 05:39
Real Name: Matthew Lacey
Location: Perth, WA

Re: Nimh in parrallel

Post by antiscab » Tue, 13 Aug 2019, 14:12

The nimh cells are out of vectrix scooters which I have upgraded to Lithium

At one point I had 12 x Vectrix batteries in my shed (each 102 x 30Ah cells)

I think i was buying problematic nimh packs back off people for $250ea (I was planning on using them in my electric hilux, but never got around to it)

I bought 20 x 175W panels and related hardware off gumtree for $1800 (about $600 was the related hardware)
panels were wired 4 series, 5 parrallel, with Vmp at 144v and Voc at 170v (depends lots on light and temperature)

4mm2 wire run from each series group down to ground level, each has it's own 10A diode, then parrallel.
direct connected to the battery.

PVE2500 inverter also direct connected - this fantastic unit can be run either grid tie or stand alone
the inverter left to it's own devices will bring the battery down to 105v. it cuts back in when the battery gets above 135v.
my problem was i had the inverter disconnected on a sunny day

I have more nimh cells, but they're in packs with bad cells so it's a lot of mucking around to use them.
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

T1 Terry
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Joined: Thu, 30 Sep 2010, 20:11
Real Name: Terry Covill
Location: Mannum SA

Re: Nimh in parrallel

Post by T1 Terry » Wed, 14 Aug 2019, 10:03

NiMh cells have a whole different characteristic when reaching fully charged, their voltage actually starts to drop and they start to accept more current and go into suicide mode. A whole lot of info about them on the Prius Chat forum and how to successfully recover a traction battery pack, but the info is still valid for all NiMh chemistry cells.
From what I've managed to gather in quick looks is, 8v in any cell pack (Prius traction battery) is relatively safe, 8.5v is asking for trouble if you are not monitoring the voltage and current very closely. Never pulled one of the Prius traction pack modules apart to see what's inside, but I guess it's a heap of cells in a parallel/series string to reach that voltage and 6Ah capacity.
What makes them stand out is the amazing ability to accept very high charge and discharge currents repeatedly, I've seen 150 amps on the scan gauge in either direction, that represents a 25CA charge/discharge rate.

T1 Terry
Green but want to learn

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