NiMH D Cell recommendation?

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TomBrown

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I'm going to make a pack to run an MR16 light. It looks like D cells are the way to go.

Are the choice brands for D cells the same as AA cells? Do the 10 Ah cells really put out 10 Ah?

My goal is to be able to drive 20W for 1.5h.

D cells: 10 Ah x 12v = 120 Wh = 6 hours

AA cells: 2.5 Ah x 12v = 30 Wh = 1.5 hours

2 parallel packs of AA cells = 3.0 hours

My concern about the AA cells is that they will sag heavily under the 1.7A load. I assume I will want to run 11 or 12 cells to get to 12 volts. Perhaps 2 parallel packs will reduce the sag.

I'd prefer to run AA cells because I can monitor and balance them with La Crosse chargers. I'm not too concerned about cost but it looks like D cells are by far the cheapest per Wh.

I'm also concerned about the quality of AA cell holders. Most seem to have 26 ga leads. If I were to go this way, I would probably want to go with one or two FM 12AA->3D adapters because of their quality. I'd use the charge tap to connect them to the light. Shape won't matter because the cells will be kept in a nylon bag.

I would appreciate any comments.
 
Thanks.

I'm choking on the thought of paying $120 USD for a dozen cells but it would be a fabulous solution so it's likely I'll go this way.

:)
 
D10000x8.jpg


10000mah NiMH D's for 8 for $49.50

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=711

I have no problems with mine...

AC
 
I've read here that the highest capacity NiMH cells won't deliver the highest current.

There are a few threads that seem to claim that for high current applications the best are CBP1650 AA cells.
 
Sub_Umbra said:
I've read here that the highest capacity NiMH cells won't deliver the highest current.

There are a few threads that seem to claim that for high current applications the best are CBP1650 AA cells.
Click to expand...

Only when talking about the same form factor. I can guarantee that NiMH D cells will stomp all over the CPB 1650s at any current level.
 
TomBrown said:
I'm going to make a pack to run an MR16 light. It looks like D cells are the way to go.

Are the choice brands for D cells the same as AA cells? Do the 10 Ah cells really put out 10 Ah?
Click to expand...

Yes, after 16 hours of charging at 1A and discharge current of 2A or less when discharged down to 1.0v assuming they're rated honestly. At C/5 rate, each cell would have a nominal voltage of 1.25v. If the 12v lamp/device is meant for using from a car, I suggest 11 cell so you'll have approximately 13.8v(voltage on car 12v system w/ engine running)

As you exceed C/5(2A for this case), available capacity starts to decrease.

My goal is to be able to drive 20W for 1.5h.

D cells: 10 Ah x 12v = 120 Wh = 6 hours

AA cells: 2.5 Ah x 12v = 30 Wh = 1.5 hours

2 parallel packs of AA cells = 3.0 hours
Click to expand...

C/5 - C/6 rate should let you achieve this with D cells, but using AA's you will not get the full capacity, because with the same current draw, your discharge rate is C/1.7.

My concern about the AA cells is that they will sag heavily under the 1.7A load. I assume I will want to run 11 or 12 cells to get to 12 volts. Perhaps 2 parallel packs will reduce the sag.
Click to expand...

2 parallel packs will more than double the run time, because you bring the discharge rate relative to cell capacity to C/3.4 which allows each cell to deliver more capacity than they can at C/1.7 rate.

I'd prefer to run AA cells because I can monitor and balance them with La Crosse chargers. I'm not too concerned about cost but it looks like D cells are by far the cheapest per Wh.

I'm also concerned about the quality of AA cell holders. Most seem to have 26 ga leads. If I were to go this way, I would probably want to go with one or two FM 12AA->3D adapters because of their quality. I'd use the charge tap to connect them to the light. Shape won't matter because the cells will be kept in a nylon bag.

I would appreciate any comments.
Click to expand...

Go D cells. It's easier to deal with a big cell than four separate parallel cells, however finding a suitable charger might be difficult. To give an effective quick charge, you will need a battery holder that accomodates however many cells you'd like to charge and a charger capable of delivering 5 to 10A.
 
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A good compromise between AA and D cells would be to use sub C cells. Check out http://www.battlepack.com/ they have loose 4300mah or ready built packs of 10 which are quite a bit smaller than D cells.

These are the sort of cells that are used in radio controlled cars so should be able to handle large current draw and fast charging. (They also sell chargers)
 
Sounds a bit like re-inventing the wheel here :).
The research seems to have been done years ago with Streamlight's Litebox that is one of the standards for many fire departments. While their application is for a 6V rather than a 12V lamp, they use a 10 A SLA battery for 8W and 20W lamps for estimated run times of 8 and 3.5 hours respectively. Given the level of draw from your application, you might want to consider trying D sized NiCDs rather than NiMH batteries; they'll probably deliver a more suitable current level and is more tolerable of weather extremes.

EDIT: Also, SLA batteries are only a fraction of the cost of NiMH batteries (using the Amondotech reference, a 12V 9A SLA battery is only $18 USD,) and NiCDs would probably be about 1/2 the cost of NiMH batteries depending upon where you get them.
 
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Brighteyez said:
Sounds a bit like re-inventing the wheel here :).
The research seems to have been done years ago with Streamlight's Litebox that is one of the standards for many fire departments. While their application is for a 6V rather than a 12V lamp, they use a 10 A SLA battery for 8W and 20W lamps for estimated run times of 8 and 3.5 hours respectively.
Click to expand...

SLAs have poor energy density and they're high maintenance. They don't like being discharged dry and they will get damaged if you don't charge up immediately after each use, whether its a car battery or a deep cycle.

If for some reason you forgot to fully charage an SLA battery and it got stored, depending on the depth of discharge, the battery is gone for good.

For fire departments, having to recharge it and keeping it on trickle charge isn't really an issue since they use it a lot.


Given the level of draw from your application, you might want to consider trying D sized NiCDs rather than NiMH batteries; they'll probably deliver a more suitable current level and is more tolerable of weather extremes.
Click to expand...

Why? His current level is less than the C/5 rates manufactures usually use for delivering spec capacity. If he goes with 10Ah D NiMH cells, 2A isn't even an issue.

I really like monkeyboy's NiMH pack idea.

monkeyboy said:
A good compromise between AA and D cells would be to use sub C cells. Check out http://www.battlepack.com/ they have loose 4300mah or ready built packs of 10 which are quite a bit smaller than D cells.

These are the sort of cells that are used in radio controlled cars so should be able to handle large current draw and fast charging. (They also sell chargers)
Click to expand...

Awesome idea. The charger is readily available and no need to manually handle 10 individual cells. 4.3Ah Sub-C huh? The cordless drill NiMH packs have 2.2 - 2.4Ah. I wonder if there's a 4.3Ah Sub-C that could be used in a high current application. (20-30A)
 
He's already mentioned that he though the cost of the NiMH batteries was a little high for him. Perhaps you missed that portion? I'm not sure if you're really famiiliar with SLA batteries or if you're just posting stuff you've read off of internet sites since you seem to be quoting theoretical maximums that may be exceed in real life usage, but this has been a solution that Streamlight has used for decades, and while SLA batteries do need to be treated diffferently they will tolerate a reasonable amount of abuse; though you're right in that they can be damaged if treated like a NiCD.

Realistically, SLA powered lights need to be topped off about once a quarter if they're not being used. Want to try that with NiMH batteries?

With regards to the high maintenance portion, I just had to replace a SLA battery in a lantern (not a Litebox) that got beat around and was regularly neglected. I'm not sure if a 7 year life under those conditions would be considered 'high maintenance'. And even when that battery was replaced, it was still registering 6.2V on a VOM.

Insofar as fire departments using it a lot, they probably use it a little more that average person who may let it sit for 3-6 months at a time, but it's not as if they're using it several times a day, or even once a day in many cases. It's not like the duty that a SL20 might go through in a police car.



Handlobraesing said:
SLAs have poor energy density and they're high maintenance. They don't like being discharged dry and they will get damaged if you don't charge up immediately after each use, whether its a car battery or a deep cycle.

If for some reason you forgot to fully charage an SLA battery and it got stored, depending on the depth of discharge, the battery is gone for good.

For fire departments, having to recharge it and keeping it on trickle charge isn't really an issue since they use it a lot.


Why? His current level is less than the C/5 rates manufactures usually use for delivering spec capacity. If he goes with 10Ah D NiMH cells, 2A isn't even an issue.
Click to expand...
 
Handlobraesing said:
I wonder if there's a 4.3Ah Sub-C that could be used in a high current application. (20-30A)
Click to expand...

The Sanyo 3600 RC pack (6 Cell 7.2V) Is advertised as having a maximum 50A discharge! So I guess running 20-30A shouldn't do any long term damage since high current is what these packs are designed for.

The SLA battery is undoubtedly the cheapest option - especially if you factor in the cost of the charger but I'm not a big fan of that steady discharge curve.

If money was no option, I would go for a LiPoly pack (again commonly used in RC models). You can't beat the energy to weight ratio but these are quite high maintainence and tend to die if run down to low and melt/explode if overcharged.
 
Hello Tom,

In answer to your original question, the CTA 12 Ah and AccuPower 11.5 Ah D cells will run for about an hour at a 10 amp load. The Powerex 11 Ah D cells run for about 53 minutes at 10 amps.

I have not tested the others, so can not comment on them.

Tom
 
Hello Monkeyboy,

Welcome to CPF.

Keep in mind that when the hobby people state that a cell is capable of 50 amps, it is under racing conditions. Fast charging and running hot off the charger.

Under "normal" conditions, the actual current capabilities are quite a bit less.

Tom
 
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