Just bought some Prius NiMH batteries!

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PriusGenIINiMH.pngI just won a bid for twelve Prius packs! They should hopefully be arriving later this week or early next.

Update: Batteries arrived and ready to be installed!

Each pack is a 7.2 volts 6.5 Ah prismatic NiMH battery with six cells. I'll be configuring them as 6S2P for 43.2 volts 13 Ah. I'm hoping that I can get at least 60% of the SOC from them (80% to 20% as on the Prius). That would be 43 volts, 7.8 Ah or 335 watt-hours.

This is perfect as I'm currently consuming 200 watts-hours each way with the heavy SLAs. The lighter NiMH packs should get my power consumption down even lower.

SLATempest TR35-12 (rated 35Ah)
-- 35 Ah @ 36 volts
-- I don't get 35Ah, closer to 10 Ah due to Peukerts

Specific Energy 35 Wh/kg nominal
Specific Energy 10 Wh/kg derated
360 Wh 1 @ 11.8 kg
3 @ 35.4 kg (78 lb)
NiMHPrius Gen II Battery (rated 6.5Ah) @ 1040 grams
-- 13 Ah @ 43.2 volts
-- derated 60% for a conservative Ah of 7.8 Ah

Module Weight 1040 g
Specific Power 1300 W/kg

Specific Energy 46 Wh/kg nominal
Specific Energy 27 Wh/kg derated
335 Wh 1 @ 1.04 kg
12 @ 12.48 kg (27.5 lb)

It looks like I'll be saving a good 20 kilograms or 50 pounds with only a slight loss in Watt-hours! I will have to bodge up a new battery carrier as the cells need to be kept under compression when being charged.

The dimensions of each pack is:

Module Dimensions 18.4 x 96 x 382 mm
Module Weight 1040 g

6S2P Dimensions: 110.4 x 96 x 382 mm
6S2P Weight 12.480 kg
Tempest TR35-12
(single pack)

Battery Dimensions: 178 x 196 x 131 mm
Battery Weight: 11.49 kg

Thoughts and comments welcome! I'm really hoping that 6S2P will do the trick although it might turn out that I'll have to do 6S3P or 18 packs to get 43 volts 19.5 Ah nominal (11.7 Ah derated).

I should mention that I'm de-rating the packs to 60% capacity. This is similar to what the Prius battery controller does (80% to 20%).

p.s. This is a really good Peukerts calculator which shows why my 35Ah battery @ 50 amps peak consumption with Peukerts number of 1.2 only gets around 16 Ah: http://www.csgnetwork.com/batterylifecalc.html

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The seller of the packs wrote in and said:

I read about your EV moped, and have a few suggestions. The Nimh batteries can be a little hard to use in parallel. You should be fine with only 2 cells is parallel, but you might want to set it up as 2 parallel strings, and place a diode on the end of each string. This way the batteries do not try to feed each other. You would also want to charge them as 2 separate strings as well.

The usable capacity of the batteries is around 80%. In the Prius they limit the capacity for a couple reasons. First they wanted a pack that would last 100k miles, and second they did not have an active cooling system. Nimh batteries can get quite warm in the final stages of charging. In the Prius, the batteries are cooled with air from inside the car, the engineers had to be prepared for batteries that might be charging with high interior car temperature, and as such would not be able to sufficiently cool the batteries if they were charged to 100%.

I use a Triton electrifly charger, and a CBA battery anaylzer on the batteries, and they usually test at 5.6-5.8Ah when discharged to reasonable limits. Also as I said before, these batteries are designed to be clamped together. If you charge them without being clamped they will bulge, and possibly be ruined.


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This page contains a single entry by Karen Nakamura published on November 24, 2009 3:20 PM.

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