NIMH BATTERIES
I'm currently testing some nimh batteries that I hope to put on sale on the
website shortly. The manufacturer has quite alot of different nimh packs that
they produce . The brand seems to be of good quality. Below are some pictures of
the nimh battery packs that I'm testing.
Below left: 24volt pack in an ebike style case, below right: 36volt pack in a an
ebike style case ( postscript: 36v case is not available anymore). Similarly to lithium batteries there is the possibility of nimh batteries catching fire, they seem to have a much safer reputation than the
lithium ion batteries. But it is still necessary to have a battery pack which
monitors the temperature of the pack when charging so as an overcharge situation
may not arise. If a pack is overcharged it may become hot and eventually catch
fire. Similarly shorting the battery packs or going above their rated maximum
amps (25amps in this case) can cause overheating and eventually fire. A 25amp
fuse is used in the pack to prevent over -current situations arising.
The 36volt pack is shown below with the cells out of the case, there are 30 'D'
size cells (33mm*56mm) in the 36volt 8ahr pack. The charger for the 36volt pack
is shown below on the right, it is a very high quality charger as far as I can
tell, CE Certified etc.
The 24volt 8ahr pack is shown below with the cells out of the case. There are 20
'D' size cells (note the 4th row of cells is not visible in the pic below the
brown rod is a piece of wood with sits inside the pack also to keep the cells
from moving around. The charger for the 24v pack is shown in right hand picture
below.
At the top of the case there is a 3 pin female connection for the charger plug
and an off/switch.Once the handle is swung down the on/off switch is not visible
sort of nice it might slow down a thief it they take a liking to your bike.
When I received the batteries they are wired up so that connection to the
controller is made by the contacts on the base of the battery.
I did not need to use this connection so I put the positive and negative wires
out of the top of the case (disconnected from the base), as shown in right hand
pic below (you can see two thick wires coming out of the top of the case).
Both the 36volt and 48volt battery packs are very similar in construction and
design, so below I will show just pictures of the 24v batteries inside the case
and how they are wired up. The top and bottom of the cases can be unscrewed to
take out the batteries. I found that the screws could be rather tight and longer
than necessary so it was a bit difficult to undo ( I need to use an impact
driver to loosen the screws on the 36volt case, the 24volt case was much easier
to undo). Once the top of the case was undone I found that there is quite a
tangle of wires under the black plastic cap and in one instance the temperature
probe wire had been accidentally broken when the cap was up in place I suspect
at the factory.
The 24volt case the batteries are a nice and tight fit in the case, with a piece
of dowel in the centre of the batteries of correct diameter so the the batteries
cant move around. The 36volt case I found that there was some rubber padding to
keep the cells from moving around, it was inadequate and the cells were quite
loose inside the pack, over time it would have been a problem and some damage
may have resulted if used as they were.
Below left: shows a picture of the cells out of the case. The 20 cells are all
joined in series, each blue segment containing 5 cells. There is a thermocouple
device which was held onto the side of one of the battery segments by some tape
(which I have removed to take the picture below right).
There is also a temperature cut off switch on the same circuit as the
thermocouple. The thermocouple was similarly joined to the top of one of the
cell segments by some tape (removed for photos below).
Also located under the black plastic top of the case is a fuse and fuse holder
the fuse being rated at 25amps (the maximum current the batteries are rated
for)(below left).
Below right shows the wiring under the top cap of the case, I think the wiring
is a bit of a mess really, and when the batteries are put in place and the lid
put on the wiring all becomes somewhat squashed together (as I mentioned one
wire was broken because of this (thermocouple wire) and a main power wire had
some of its outer plastic stripped off leaving an exposed positive main power
wire).
Pictures below show the 36volt batteries out of their case, the large pieces of
black rubber are designed to keep the batteries secure, but the rubber had moved
out of position on the batteries were very loose .
Below shows a closeup of how the cells are connected. At each end of each cell
are metal tabs to which thick copper wire has been soldered. The soldering was
all good and some heat resistant tape was put over the top of each tab. But I
did find that whilst i was messing around with the cells that if the tabs were
straightened and then flattened again a couple of times they would crack along
the fold line of the tab, so I had to be careful not to mess with the tabs too
much.
Each charger has the same type of plug, a good quality 3 pin connection
type, so theres no chance of puting the plug on incorrectly.
The 36volt charger seemed to be a very good quality charger, somewhat larger
than the charger used with the 24volt batteries.
Pictures below show the two ends of the 36volt charger. (postscript: both the
36v and 24v batteries are now with the same high quality charger ).
Picture below shows the 24volt charger ( postscript: not using this charger
any longer) It has just one led, when the charger is
charging the led is red in colour, when the batteries are fully charged it turns
to green. The 36v charger above has two leds, one shows that mains power is
connected and remains red. The other led is red when charging, orange when 90%
full and green when the battery pack is fully charged.
Note: Only the 24v batteries come with cases ( factory making the cases
doesn't make the 36volt cases anymore, so for time being I cant supply 36v nimh
in aluminium case only the 24volt version can come with aluminium case for time
being)
A bit about the batteries:
The manufacturer makes alot of different packs. I think only 24volt and 36volt
packs are available (I have to check about 48volt packs), but the 24v and 36v
packs can come in sizes from 8ahr up to 20ahr. I think it would be quite
possible to use two 24volt packs in series to get 48volt s (if manufacturer
doesn't make 48volt packs).
[ I tried puting two 24volt packs in series to run at 48volts but for some
reason the controller wouldn't work, I haven't figured out the reason as yet,
but suspect voltage may have been too high and controller was cutting off, have
to resolve that in near future]
Maximum Current rating for all the packs is 25amps. That quite high and
will suit most ebike applications.
Constant Current rating for the batteries is 5amps. That seems quite low
to me, I was expecting more like 10amps. I'm hoping that either Shen (my
assistant in china) has made a mistake or that they have put a large safety
margin with quoting that figure. On a typical ebike the batteries may be
required to supply 8-10amps constantly, and up hills that may go up to 20amps or
so. So for typical ebike use it looks like the constant current value will be
exceeded regularly. Whether or not that will have a significant impact on the
life of the batteries I have no idea. I could only hope that the 5amp rating for
constant current is a conservative figure from the manufacturer and that it can
be exceeded for periods of uphill or travel on flat roads with little or no
pedal assist. So as you can see I need to do quite a bit of testing. The
manufacturer of course has done alot of testing on the batteries but they would
be doing under lab conditions probably and using the batteries on ebikes would
require some extensive period to see how many life cycles the batteries will
last for with typical or even harsher ebike riding.
Low voltage cut off for the 24volt battery packs is 21.5volts.
I think most controllers used with sealed lead acid batteries at 24volts will
cut off at about 21.5volts so there should be no problem using with standard
controllers.
Low voltage cut off for the 36volt battery packs is 30.5volts,
once again standard controllers for sla batteries should cut off at about
30.5volts for sealed lead acid batteries, so these batteries will suit standard
controllers.
The only issue may be that voltage sag of the batteries when under load may take
them down to a level close to the cut off voltage of standard controllers (with
a low voltage cut off set to suit sla batteries), at present from some testing
that doesn't seem to be the case, but I need to do some more testing on voltage
sag on different motors to confirm if that will be a problem or not.
Cell battery size: the cells appear to be "D" size
(33mm*56mm), I wasn't sure what size they were but seems they are close to
"D" size except they appear to be 56mm long rather than 58mm long but
that could be due to being wrapped in plastic is a bit hard for me to measure
length accurately. Or maybe as they have tabs rather than normal style battery
ends.
Prices for different packs with the higher quality chargers are:
24volt 8ahr (with charger): $250aud (plus $70aud freight) = $320aud (comes
in aluminium case)
24volt 10ahr (with charger): $280aud (plus $70aud freight) = $350aud (comes in
aluminium case)
36volt 8ahr (with charger): $305aud (plus $90aud freight) = $395aud (no case
company stopped making the 36v cases)
36volt 10ahr (with charger): $335aud (plus $90aud freight) = $425aud (no case
company stopped making the 36v cases)
Test Results thus far:
1.I'm testing some 24volt packs using the P2a motor on 24volts, just waiting
for a drain brain meter to arrive before I put some results up.
2. Chris P. of Western Australia has been kind enough to keep a record of
his use of a 36v 10ahr nimh pack, below are some results he's got thus far: He's
using a 408 crystalyte motor on 36volt in a 26" rim.
beware of spray ebike company: more info
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