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NiMH Battery Pack Substitution Effect on Charging Circuit

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ogden:

--- Quote from: edy on October 31, 2018, 10:17:17 pm ---Thanks for the help, yes I agree it could be an issue... I don't want to wait that long to charge the battery.

--- End quote ---

You can use battery half-charged, but then what's the point to have such a big capacity and what's worse - it's (dead) weight that slows your RC car down? [edit] What are charge time specs for that toy? - User manual shall tell about charging at least something.


--- Quote ---Another option that I was thinking is to use one of those 5V Li-Ion cell-phone power packs. The car takes 4.8V but if I feed it a 5V output from a Li-Ion cell-phone charger, will it really care?

--- End quote ---

Car could be slower with 4.2V Li-Ion than fully charged 4.8V NiMH, but it will run indeed. On the other hand LiIon pack is lighter for given capacity, so car will benefit from it.

Rick Law:

--- Quote from: drussell on October 31, 2018, 01:29:20 pm ---
--- Quote from: Rick Law on October 31, 2018, 01:45:38 am ---A 2200mAH battery may draw too much charge current - more than the charger circuitry can handle.
--- End quote ---

:bullshit:

A completely discharged 600 mAh pack will look like a very low impedance load, just like a 2200 mAh pack will.  It is the charger that limits the current, not the battery pack.  Many simple chargers are just a resistor from a voltage slightly above the Ni-MH "full charge" voltage. 
....

--- End quote ---

[Emphasis added]

Geeze, I thought the one I found (on a cheap hand-crank charging "emergency" radio) that was just a resistor (plus a diode) was unusual...  Amazing how cheap can things get really.

You are probably right that discharged battery packs would look like a low impedance load.  But just the same, given all the other draw-backs, I won't risk it.  You know, one missing piece of info is the charge current...  May be that guy is just drip-charging it at 20mA or some such, and without termination at all.    All the more reason not to use that to charge better batteries.

[EDIT...  adding this thought below:]

While I said I wouldn't use that to charge better batteries, don't interpret that as suggesting one should charge the batteries individually.

That is a 4xAAA pack, as a pack, the pack needs to be balanced.  If the cells are charged individually, some would have more charge than others.  During discharge, the lesser cells may "go negative" after their lesser charge is gone and the better ones are still pumping out juice.  When a battery eventually "gone negative", that is the death for the battery.

Audioguru:
Edy, you did not lookup the charging voltage of a Ni-MH cell. The Chinese manufacturer printed 4.8V (1.2V per cell) because 4.8V is its discharging voltage.
The charging voltage is way more than only 1.2V per cell.

The charger circuit must be smart enough to limit the current and to disconnect charging when it detects a full charge. An overcharging battery gets hot and might burst.

The charging voltage is shown to be more than 1.4V per cell (more than 5.6V for your 4-cells battery) on Energizer and Duracell datasheets:

Rick Law:

--- Quote from: edy on October 31, 2018, 03:11:43 am ---Thanks for the input.
...
...
Anyways, the car is $10 now, but originally it was priced around $99. I don't know if that necessarily reflects that the folks at Garmin well-engineered it, or anticipated that someone may try to screw around with the batteries. Nevertheless, I do have AAA's sitting around as well which I could try out but meanwhile I will try an over-night charge of my 4xAA pack and hopefully it won't burn out any of the circuit.

--- End quote ---

The way you have it rubber-banded to the topside made me think... if I may make a suggestion on an entirely different approach...

How about those small USB power banks?  The power bank odd to be able to rubber-band to the car the same way your picture shown.

Yeah, USB power bank is going to output 5V+-5%'ish.  Your original battery pack is 4.8 volt, that is 1.2v nominal for each NiMH cell.  But NiMH cell is not exactly 1.2v always.  On fresh charge, they are going to be 1.3v or 1.4v or perhaps even more.  So your 4.8v pack is going to be 5.2v or 5.6v or more when freshly charge.  That is just within the USB power-bank's expected output.

My ASUS power-bank has 3x18650 in parallel (10050mAH total), boosted to output 5v, smaller than a pack of cigarette.  You have all the charging solved for you along with over 7000mAH @ 5v.  That is a good bit more than your 2000mAH 4xAA cells.  With that, you can race all night with it...

I got mine direct from ASUS, but B&H also has it:
https://www.bhphotovideo.com/c/product/1173356-REG/asus_90ac00p0_bbt002_zenpower_10050mah_portable_battery.html

If you like this direction, check out this type of power banks (this types, not this one):
https://www.aliexpress.com/item/Multicolor-New-USB-5V-1A-POWER-BANK-Suit-18650-BATTERY-External-DIY-Kit-Case-Box-Per/32760647698.html
As I said, this types, not this one.  Look for a better quality one as this one is crap.  I actually got a couple of this exact one for experimentation.  The boost board is crap, but it works.  At 1A, it appears to run well but rather hot.  This one is sized for unprotected cell.  You can't fit a protected 18650 in it.  While it works well for discharge, I wont charge the 18650 with it.  I got two of those and  I ripped apart one for experimentation.  It has charge voltage that doesn't seem right.  I have plans to test the other one.  I have not had a chance to check the second one.

IanB:

--- Quote from: edy on October 30, 2018, 01:26:04 pm ---The question is.... If I leave my newly made battery pack hooked up to the car and use the charger built into the car instead of taking them out to charge, will it cause any issues? Remember, the battery pack originally in the car (that never comes out) is 4xAAA in series giving 4.8V and 600mAh rating. I will instead have 4xAA in series, also 4.8V but probably in the 2000mAh range. Is it just a matter of taking longer to charge, or could it overload the charging circuit in the car?

--- End quote ---

How long do the instructions say it should take to recharge the built-in battery pack? It is very likely that it is just a simple fixed current charger. A fixed current charger would probably take 6-10 hours to recharge. If the cars are supposed to recharge in 4 hours or less it may be a smart charger that is designed to stop charging when the batteries are full.

Now then: the risk of overloading the charging circuit is close to nil. Not much risk at all.

If it is a dumb fixed current charger then there is also no risk of damaging bigger batteries. However, be aware that if it takes 10 hours to charge a 600 mAh battery pack it will take over 30 hours to charge a 2000 mAh battery pack. You may not want to wait that long.

If it is a smart charger it will also take 3x as long, but there is also a risk that the end of charge detection won't work properly with the bigger batteries and it doesn't stop charging when the batteries are full. On the other hand, since the batteries are so much bigger they will absorb the extra charge with less risk of harm.

A faster option would be to remove the batteries and charge them externally, but you have to multiply this by the number of cars and the number of chargers you have.

Using a lithium ion power pack is a possibility, but it might be overloaded by the cars and might perform less well than the NiMH batteries. You would have to try it and see.

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