Can I use a car battery charger?

I have a Mag85 using 9AA cells.

I don't currently have a good battery charger that can charge individual batteries. This obviously leads to a problem of an odd battery.

I was thinking about all the things that I have for cars and my four wheeler.

I first measured the voltage on a regular car battery charger. That reads 16.5V. A little too high.

I then measured my battery maintainer. It read 13.8v. I think this is perfect.

The battery pack measures 13.05V, 1.45 per cell, hot off the charger. I know that you need more input voltage to overcome the internal resistance on the battery.

The charger is rated at 1.5amps normal charge. I don't know the trickle charge rate.

Is it ok to use this charger?

Is the voltage put out enough to top off the batteries?

Am I going to blow something up?

Is the 1.5amps too much?

Thank you for your help.

Do you have a multimeter with a 10 A scale? What is the capacity of the battery pack?

The short answer is that you can charge that pack using a timed charge as long as you keep the current below 0.1C when the pack is fully charged. For a 2000 mAh pack that means keeping the current below 200 mA or 0.2 A. A possible way to do that is to put an automotive bulb in series between the charger and the pack. You could experiment with a 5 W or 3 W bulb and use your meter to check the current. Once you have the appropriate current sorted, let it run for 10-15 hours. This will also balance your pack for you, which is good.

Another test for end of charge is to see when the pack starts getting warm. You would charge until any of the cells start getting hot and then stop.

As you can see, you need to apply some measurement and control to the process. You cannot just connect up a car battery charger and hope for the best.

Why do you recommend such a low charge rate?

Most people recommend a 0.5c to 1.0c charge.

I am using Duracell 2650 cells. So I'm running at 0.6c charge with this setup, right?

I do have a multimeter with up to 10amp reading.

poormanq45 said:

Why do you recommend such a low charge rate?

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So that you don't blow up your battery pack.

Most people recommend a 0.5c to 1.0c charge.

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That is only when using a "smart" charger specifically designed for NiMH batteries. When you try to use a car battery charger you are effectively using a "dumb" charger and you must adapt your charging protocol to suit.

I am using Duracell 2650 cells. So I'm running at 0.6c charge with this setup, right?

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I don't understand the question. However, for a recommended 0.1C charge on this pack you must limit the current to about 250 mA or 0.25 A.

I do have a multimeter with up to 10amp reading.

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That's good. Then as long as you control the charging current through the pack within safe limits you should be OK. But I recommend you not to try this unless you are prepared to buy nine new cells if you should get it wrong and damage your pack.

I have to ask, what is the difference between a single cell vs a pack?

When they're all in series I know voltage adds, but total capacity is the capacity of one cell, correct? Or am I missing something?

What I was going to do is just measure the voltage that the pack gets to. This seems to be an accurate method to determine charge, relative to using a "smart charger". Using the smart charger each cell gets to ~1.45v. The pack is at 13.05V. This settles down to ~12.6V after about 1~3 hours.

If I monitor the pack and disconnect it at about 13v will the cells be fully charged?

When it comes to current, why would it be recommended to charge a single cell at 0.5c to 1.0c, but not charge the pack at that rate?

Thank you very much for helping me.

With a single cell, the voltage across the cell terminals tells you about the state of charge of the cell.

With a pack, the voltage across the whole pack does not necessarily tell you the same information. You cannot be sure about each individual cell in the pack even though you know the sum of all the voltages. You can measure 13.05 V, but some cells might be more than 1.45 V and some cells less. This can happen if the cells have unbalanced starting charges or if they have aged unevenly. In order to avoid damaging any cells you have to keep good control of the current through the pack, not just the voltage. Otherwise it is very easy for one cell in the pack to be overcharged and damaged. (To make this clearer, 1 A flowing through a discharged cell is not going to hurt it, but 1 A flowing through a charged cell will damage it. Since the whole pack gets 1 A, and since you don't know all the cells are evenly balanced, you have a problem.)

The recommendation about 0.5C to 1.0C charge rates compared to 0.1C charging is not about the difference between a single cell and a pack. It is about the difference between a smart charger and a timed charge. You can charge a pack at 0.5C to 1.0C too if you have a smart pack charger e.g. a hobby charger like this one: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=7028

You do not have such a charger, so you cannot charge at 0.5C safely.

That definitely makes sense.

All the batteries in the two packs I have have been used together, charged on the same charger, and run in the light together. I'm fairly certain, not figuring in defects, that these cells are very close together in terms of charge and life.

I've got 24 of these batteries, so I have a few to spare.

For fun, and interest, I put the pack that measured 11v on the charger. I made sure to put it in a safe spot, outside on concrete in a cardboard box, and monitored the voltage and heat. After about 1 hours the entire pack reach 119 degrees F as measured by an IR laser. The voltage read 13.11v. I disassembled the pack and measured every cell. The range variance was 1.45 to 1.46.

I take it that this means that these cells in this particular pack are pretty well balanced. Is that a correct assumption?

Thank you very much for your information. I understand your advise takes safety as a major factor. I do things safely, but I also like to push things.

Ok, I think I just found an issue with my setup.

I was thinking that the internal resistance on these batteries would be higher. I'm not showing any resistance through the pack!

I think 13.8v from the charger may be too much. That would put me at 1.5v per cell. Is that too much?

What is the voltage output from a single cell charger?

*edit*
Alright, I found the trickle charge.

At 13.8v it is 0.136a.

As far as safety is concerned, NiMH cells are not going to catch fire or anything; but they can vent gases and/or overheat and melt their plastic covering, suffering irreversible damage in the process.

Cool, not as bad as Li-Ion :-D

The question is not what the trickle rate is (well it is, but not yet), it is at what voltage does it go into trickle mode. There exists a possibility it could stay at full charge rate because it expects the cells to rise higher than their peak charge would be. You may have already ruled this out, it was not clear what you observed through measurements and when.

Thank you guys very much for all the information.

I realized, after reading alot of information, that in the long run it'll be better to have a proper charger.

I was out and about today and stopped in to HobbyTown. I picked up a Venom Pro Charger. It seems to do everything, and more then the one listed. It also includes a power supply.

It's a lot more expensive then the Turnigy listed, but it was available locally and I am impatient. I've currently got the battery pack discharging.

What is the recommended lowest voltage for NiMHs? The manual for these thing suggests 0.9V. So I set it to discharge at a rate of 0.5amps until 8.1V is reached. Is the the proper setting?

Again, thank you very much for the information.

Well, I'm officially anooyed. I checked out the charger online.

I could have gotten this one for alot less http://www.virtualhobbyshops.com/product_info.php?currency=USD&products_id=338 . Its the same thing... Ugh

poormanq45 said:

Well, I'm officially anooyed. I checked out the charger online.

I could have gotten this one for alot less http://www.virtualhobbyshops.com/product_info.php?currency=USD&products_id=338 . Its the same thing... Ugh

Click to expand...

But that one doesn't have a power supply -- it requires an external 12 V power source.

poormanq45 said:

Well, I'm officially anooyed. I checked out the charger online.

I could have gotten this one for alot less http://www.virtualhobbyshops.com/product_info.php?currency=USD&products_id=338 . Its the same thing... Ugh

Click to expand...

That is essentially the same charger as the one that Mr Happy posted a link to. Same features, same specs, same connectors - no meaningful differences.

Yeah, I looked at them.

What power supply would you use with them?

poormanq45 said:

Yeah, I looked at them.

What power supply would you use with them?

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A free one! I use an old computer PSU - easy to mod to turn on and easy to tap off 12V (yellow = +12V, Black = Ground).

What supply you could get away with using (as far as a minimum goes) depends on the charge rate you need and whether it has a linear or switching output circuit to charge. Given the wide input voltage range it is probably the latter and could potentially charge a series of AA with nothing more than a 12V, 1.5A wall wart but I am not certain of that, a little laptop brick supply seems the ideal in small size and cost effectiveness if you need to buy something.

I didn't even think about the computer power supply.

I always have a few laying around. 12volt rails ranging from 13amps to 93amps

I'll take that in to consideration if this charger dies in a year or two.

Now, back to one of my questions. What level should I discharge the cells to?

I tried getting down to 8.1v, but the charger only discharges at a minimum of 0.1amp. The pack couldn't even sustain this draw without dropping to 1v. Once the draw was removed the pack went back to 9.5v. What is going on?

poormanq45 said:

Now, back to one of my questions. What level should I discharge the cells to?

I tried getting down to 8.1v, but the charger only discharges at a minimum of 0.1amp. The pack couldn't even sustain this draw without dropping to 1v. Once the draw was removed the pack went back to 9.5v. What is going on?

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Well, 1 V is way, way lower than 8.1 V, isn't it? Doesn't the charger automatically stop the discharge at the voltage you set? How does it even get down to 1 V?

For a pack with many cells it is often recommended to discharge to 1.0 V per cell, or even 1.1 V per cell. This is the voltage under load. As soon as you remove the load the voltage should gradually rebound to about 1.2 V per cell. If it doesn't, the cells are damaged.

Remember that deep discharge of a pack is potentially damaging. Don't try too hard. Discharge down to 9 V and stop, don't keep discharging further.