# 20 of 10000mAh NiMH D cells for 500W motor

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hp79
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20 of 10000mAh NiMH D cells for 500W motor

I just bought a Schwinn s500 electric scooter.
24V 500W motor, two 12V 10A lead-acid connected in serial.

I'm worried about the lead-acid battery, read that it shouldn't be discharged too deep. I'm thinking of replacing the battery to a NiMH type so that I can use almost all capacity every time.

Can I use 20 of D size cells (10000 mAh) all connected in serial, to make a 24V 10A battery pack?

I read some equations like 500watt/24V = 21A, so that I would need a power system that can supply 21A continous current. Does this sound right? If so, does that mean the D cells can only produce a current of 10A, so I'd only be able to use half of the 500 watt motor power?

How about using 20 of Ni-Cad?

Thanks in advance,

Hoon

HCT
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Re: 20 of 10000mAh NiMH D cells for 500W motor

What you are proposing is very logical , your 500 watts may have momentary much higher peak power drawn , would not be suprised to see 40 to 60 amps peak surge .
Recommende taking a close look at the internal impedance of those 10A/H cells ,anything over 5-6 milliohms and you are throwing away quite like of energy . I have worled with D cells by Panasonic " HHR650D " rated at 6.5 a/h they are about 2.0 milliohms impedance .
If you expect to bring down the Cell to close 100 % DOD , would recommended that you matched the cells , although have found the HHR650D stock cells to be within 1%-2% incrediably good ( purchased from Digi key )
The one down side is their cost about \$15- \$18 in OEM lot.
In summary get a impedance reading on the cells that you considering , the lower the better , have seem "D" 10A/H with 15 milliohms . they are suitable for nice slow drain less 2 Amps , they would not work at all for the 20 A + required by a 500 W motor load .Have not been able to find " D " cells with lower impedance than the HHR650D

HEAT & LOSS & INEFFICIENCY RELATED DIRECTLY to IMPEDANCE

andrew
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Re: 20 of 10000mAh NiMH D cells for 500W motor

If you are considering the Chinese D cells, then that may be too much for them from what I've heard. Also, the chinese cells don't perform to their rated capacity especially at high rates.

I would just use the lead-acid. There is a lot of misinformation on the net, especially from forums like these. People will claim that you will kill lead-acid AGMs quick if you discharge them deeply, but they don't reference any manufacturer testing to back that up.

From the spec sheets from B&B batteries, discharging deeply is not a problem. As long as you don't discharge beyond about 10v you are fine. Once the batteries drop to 10v then they are almost fully discharged and will drop real quickly, so beyond that is a risk of reverse charging a cell. But, when you get to that point, you've probably used >95% of the capacity depending on the discharge rate. And the controller may have a low-voltage cutout to protect from that. If it doesn't, then just stop running the scooter when it gets noticeably slower.

A greater risk of damage is balance problems. Even a two-battery string may get out of balance. I would get two 12v chargers and charge each battery separately, or put in a double pole double throw switch to switch the batteries in parallel and charge them with one 12v charger.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

hp79
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Re: 20 of 10000mAh NiMH D cells for 500W motor

Thanks for the advice. About the life cycle on lead-acid, I saw a data that was provided by the manufacturer. If I use up 75% of battery at a time, it will only give 200 times of life cycle. If I use only 25%, it gives something like 1000 cycles. I also don't like the fact that lead-acid's have to be recharged right after use to prevent going bad.

I asked one of the technicians at all-battery about one of their product
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=442
, and here is the part of the response.

"This pack can handle 40Amps continuous current which has not problem to handle 21Amps requirement of your scooter.
Kevin L.
All-Battery"

Their battery pack is made of 20 tenergy brand subC NiMH cells. (24V 4200mAh)
If this would work, why wouldn't I build my own using 20 size C 5000mAh string so I can go cheap?

Schwinn S500

andrew
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Re: 20 of 10000mAh NiMH D cells for 500W motor

That looks like it will work. The sub C cells will probably have an equal or lower impedance than the larger C chinese cells.

If I was to do this, I would go with a high quality manufacturer. SAFT makes some D size NiCad cells which are 5000 mah. It may be more difficult to fit 20 of these, but the quality, life, and performance will most likely be a far cry from the chinese cells. Check the VRE D 5500 towards the bottom here:
Batterystore SAFD D Nicad

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

hp79
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Re: 20 of 10000mAh NiMH D cells for 500W motor

I checked out those D size Ni-cad batteries you recommended. They look nice, and I think I'll go with those.

Making a long string of 20 cells, is there an easy way to put on some kind of a sensor for each cell so they don't go lower than 1.0V? I'm afraid of the cell reversal especialy having all 20 in one string.

Also what would be a good way to recharge the batteries? I thought of making two strings of 12V, and use two 12V smart chargers for each string, for its simplicity.

I tried to google how to make a protection for such long string, but I didn't really know what to search for. I did read most of the material on batteryuniversity website.

Schwinn S500

andrew
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Re: 20 of 10000mAh NiMH D cells for 500W motor

I don't know of an easy way to monitor the cells for low voltage. If a cell gets reversed because of having less capacity, you can always test for that and replace the cell. A 20v string voltage cutoff should provide fair protection.

That is an interesting problem though. I'm sure there is an integrated circuit (IC) that could provide that functionality. Maybe even one available at Radio Shack. The way I would set this up is have it cycle through all of the cells sampling the voltage. Try and get a high enough sampling rate so that it cycles through all of the cells >5 times a second. Once one drops to <1v then have the IC trigger an automotive relay. I think some of these can handle switching 30-40 amps. The circuit to switch the relay would need to be amplified as the IC could probably not handle the power needed to control the relay.

A more advanced approach may be to have the IC send a reading of the lowest cell voltage to a digital meter that you can read with fair resolution like down to 10 mv increments. then once this drops to less then 1v, you would know to stop. Or both systems could be used having a display and an auto shutoff. This would no doubt require a fair amount of programmability from the chip.

To recharge the batteries, two strings of 12v would be a cautious approach. I would definitely recommend doing that if you wouldn't mind going through the trouble to set that up.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

Mik
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Re: 20 of 10000mAh NiMH D cells for 500W motor

In summary get a impedance reading on the cells that you considering , the lower the better

How do you do this??

Mr. Mik

This information may be used entirely at your own risk.

There is always a way if there is no other way!

HCT
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Re: 20 of 10000mAh NiMH D cells for 500W motor

Hi Mik

Cell impedance (or internal cell resistance ) can be measure several ways , the one that I prefer is the standard that most battery manufacturer use .
1 Khz AC milliohmmeter such as HP4328A or HP4338 series is the instrument refer to when a impedance figure is provided .
There is a DC method that can be used to calculate( NOT MEASURE )the impedance of a cell . however have found that the charging or discharging current used in the DC method is not as quick and depend more on state of charge , temperature and other variables .Measurement in both method are fairly close (within 10-15 %), my preference is the AC method since it is very fast and repeatable .
The actual impedance number is of lesser importance than the numerical change between charge and discharge state of a cell or impedance cell variation in one production run .
AC impedance measurement is very useful when comparing similar size and amp/hour cells from different manufacturer .
Believe that impedance it is far more important than A/H figures when working in High C discharge application ,when used in low discharge .
There is direct correlation between low impedance and A/H capacity ,so in a group of identical cells the lower impedance ones will most likely the higher capacity.
The one exception that I have found , some cells have internal partial short ,they will have lower impedance , however since they are self discharging , their A/H rating will be much lower .if the A/H test is perform right after charge (within a few minutes ) , it will not be as apparent and completly missed .In my 40 years working with power generation , storage ,conversion and management have found the most important factor is impedance , since it is the one directly influencing efficiency .
Have found most batteries user either do not understand or accept the importance of impedance , have been reading comment on this and other site .Impedance is very rarely used ,like it was a dirty or curse word .
Without any question ALL BATTERIES FAILURE ARE ALWAYS IMPEDANCE RELATED ...........

HEAT & LOSS & INEFFICIENCY RELATED DIRECTLY to IMPEDANCE

dogman
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Re: 20 of 10000mAh NiMH D cells for 500W motor

Bear in mind that lead may work fine for you if the battery has the range for your needs. If the bike sits all day uncharged, that is an issue too. But while lead does have it's inconveniences, it is still a cheap way to go. If you do a deeper than 80% discharge once a weekday, you still get almost a year of riding at 200 cycles. If you have money and just want more range and convenience you would love a good sized lifepo4, like 20 ah.

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

hp79
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Re: 20 of 10000mAh NiMH D cells for 500W motor

Thank you for all your advices everyone. I'm learning a lot from the forums.

I decided to go with Chinese 20*D size pack (5000mAh) Ni-Cd for \$80. I was seriously considering Saft quality cells, but the price of \$132+ made me buy the cheap Chinese Tenergy.
I was also considering Sanyo (3600mAh) C size cells which would've costed \$101.

I learned to avoid the cheap Chinese NiMh D cells because of the internal resistance. Then I thought since NiCd in general has a low resistance, even though it is a cheap Chinese battery pack, it might work good for my scooter.

I'm planning to fully discharge these once a month, but scared of the voltage reversal that could damage the battery. So I might just punch 40 holes on the pack, connect resistors for each cells when I want to fully discharge to 0.5V. Is there any other way to fully discharge the whole pack safely?

Here's the link to what I ordered.
20 D size Ni-Cd tenergy cells, 5000 mAh pack.
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=436

----edit June 16, 2008----
I canceled the tenergy battery pack because it doesn't support high current draw.
After placing the order, I just called them to make sure that this will work for my scooter. The tech guy there told me even though it's a ni-cd battery pack, it is only for 10A and below.
So I ordered 20 tabed D cells (VRE Nicd 5000 mah, Saft) from batterystore. I hope this will work out well.

Schwinn S500

needWheels
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Re: 20 of 10000mAh NiMH D cells for 500W motor

From everything I have read, you'll never get more than 1C from D cells (meaning rated capacity, ie. 5000mah = 5ah no more)

Go sub-c and you can get 5-10C.

I've been looking at 10 x 7.2v RC packs (36v x 2)

chas_stevenson
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Re: 20 of 10000mAh NiMH D cells for 500W motor

When I was racing RC cars we used a simple device called a "battery bug" which was plugged on our 7.2-volt battery pack and left on until the next charge and use. We used a normal LED and a 1.2k Ohm resistor in series. When the battery bug was connected to the battery it would take several days to discharge the battery down to the point where the LED would go out. Because the LED required 1.2-volts to forward bias and come on the battery was never discharged to a point lower than 1.2-volts and the cells would NOT reverse if left on too long. This worked well for NiCad packs and if you made enough of them for your pack you could place 1 on every pair of cells and because the discharge is so slow the cells would be discharged to .6-volts without the fear of reversing the cells.

Grandpa Chas S.

hp79
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Re: 20 of 10000mAh NiMH D cells for 500W motor

On the datasheets of Saft D size NiCd battery, it shows plots of up to 40A discharge rate, and still doesn't decrease much in capacity. Isn't 40A like 8C? I think what your talking about is low quality Chinese NiMH battery.

Saft VRE D 5500 cells has continuous 50A, 0.3 sec peak 150A on their datasheet.
I think Saft is a reputable company, so I trust their datasheets.

Schwinn S500

chas_stevenson
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Re: 20 of 10000mAh NiMH D cells for 500W motor

hp79,

You are right we are not talking about Saft Batteries. The ones we are talking about are lower quality.

Thanks,
Grandpa Chas S.

HCT
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Capability of NiMH D cells

Regarding "D " cells I have assembled a cell using a Pansonic HHR650D 6.5A/h welded 10-23 post , was able to take out over 300 AMps into a dead short for almost a full minute .Those cell have 2 milliohms impedance when fully charged The peak current that you able to take out of those cell is E/2R or 300 AMPS or almost 50C
Have to see a "C" or any other size cell of any chemistries matching this .
Love to heard otherwise .As before keep going back to impedance .................................lower the better .

Several years ago assembled a 36 volt 5000 Watt/hour pack for a electric plane.

HEAT & LOSS & INEFFICIENCY RELATED DIRECTLY to IMPEDANCE

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