The original Prius (NHW10, the one only sold in Japan) used 2 different
thermistors, there is a string with 1 thermistor per cell and another 4
thermistors per whole 1/2 pack of 120 cells. The 1 per cell thermistors work
very suddenly, almost like a switch and are all in series so are monitored
as a whole. All you know out of that is if any cell is above about 55 deg C.
The other 4 are monitored individually and are spread through the pack.
These are pretty linear in response and tell the computer what the
temperature of the pack is.
The cells are Panasonic HRR650 "D" cells. This is the Panasonic
manual.
http://www.panasonic.com/industrial/includes/pdf/Panasonic_NiMH_ChargeMethod
s.pdf
I think it's these: http://www.mouser.com/ds/2/418/tyco_srp120-204974.pdf
Not sure which one, but they measure about 1 Ohm at room temp.
-----Original Message-----
From: EV [mailto:[email protected]] On Behalf Of Lee Hart via EV
Sent: Monday, 2 June 2014 8:08 a.m.
To: Lawrence Rhodes; Electric Vehicle Discussion List
Subject: Re: [EVDL] Thermistor to use with 7cell Nimh pack.
Lawrence Rhodes via EV wrote:
> I'm building a small 7 cell pack with two strings (14 total) of Nimh
> 4ah. I'll be using a generic 10 buck charger designed for 7 cells.
> 350 ma. I need to protect the pack. Is there a specific Thermistor
> that triggers at the right voltage to protect the pack?
You don't want to charge nimh cells in parallel. As they approach full,
their voltage *falls*. The falling voltage of the fully charged cell
prevents the one in parallel with it from reaching "full".
The simplest way to get around this is to add a series resistor (or
something with resistance) in series with each string. The voltage drop
across it allows each string to independently reach "full". However, then
you have to monitor each string separately, to know when it reaches full, so
you know when to stop charging.
A 10-dollar charger probably has no brains. It probably mindlessly applies
some "trickle" current forever, and never shuts off. Nicads could tolerate
this, but it is bad for the life of nimh cells.
The best algorithms for charging nimh cells involve looking for the voltage
peak near full charge, and then shutting off. Or, some use a sensitive
thermistor to sense cell temperature, and shut off when it begins to rise.
Or, count amphours, and stop when you have put back 110% or so of the charge
removed.
--
I view this year's failure as next year's opportunity. Failures are not
something to be avoided. You want them to happen as quickly as you can, so
you can make progress rapidly. -- Gordon Moore
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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