Charging Four (AA) battery using a car battery

[thomasPlam]

Hi guys,

I planing to install some alarm in my car, which is operated by a four AA rechargeable batteries (1.2v 2500 ma). I am planing recharge it with my car battery(12v). I want to know few things before doing this, my query is given below.Please help me.


1) To recharge the battery at 500 ma it will take some 5 hours to fully charge, as per the calculation.So whether it harm my car battery ?

2) I am going to use a transistor, to switch the 12v source to my LM2940(low dropout) input because my entire circuit works in 4.8v. Is their any problem if i use transistor to switch? Relay i cannot use because of space problem.


Any suggestions will be appreciated.

regards,
thomas

 

[Nigel Goodwin]

Why would you need to charge it so fast?.

Presumably it's permanently connected to the car?, just trickle charge the AA's to keep them topped up.

Or power it from the car battery entirely, without the AA's (but a trickle charge to the existing batteries would be very cheap and simple).

 

[jpanhalt]

1) I can see your need for a separate battery within the device.

2) Your calculation is accurate as a first approximation. There are, of course, losses from heat. Most important, you will probably not want to totally discharge the AA (NiMH?) batteries for each cycle. I agree that periodic recharging of partially discharged batteries is probably best. As for continuous trickle charging, some battery chemistries are not hurt by that (e.g., lead acid) and others may be. You may want to review the manufacturer's recommendations for your specific brand and chemistry of batteries. I am not referring to the so-called "memory effect" of NiCd batteries, but rather to limits on the number of charge-discharge cycles of other chemistries.

3) Regarding harm to your car battery, remember that that battery is designed for starting. It is not designed for delivering continuous power over extended periods without recharging. Deep cycle/Marine batteries are designed for the latter purpose. Usually, the starter battery is only discharged a small percentage of it total capacity. If that battery is only 20AH, then charging that AA pack will take about 13% of its capacity. That may be OK, but you have to consider that it will also be called to start the car. What size battery is in your car? Here is a link to battery University that goes into such discharge effects in more depth: **broken link removed**

John

Here is some additional information and links with comments:

https://www.greenbatteries.com/bachfa.html#Is trickle charging harmful? (Recommends against continuous trickle charging of all NiMH batteries. Recommends "pulse charging.")

https://www.electro-tech-online.com/custompdfs/2012/09/Panasonic_NiMH_ChargeMethods-1.pdf

Trickle charge is not recommended for NiMH batteries.
<snip>
Pulse charging can be used.

I switched to Sanyo eneloop several years ago because of its low self-discharge rate. For example, I can charge a set of 4AA batteries in October and by May/June, they are still at least 90% charged based on the mAH my charger used to charge them to "full capacity." I did not find whether Sanyo recommends against continuous trickle charging. I do not do that. The low self-discharge may be advantageous for an alarm system. Some competitors are now making similar claims. I have not tried their batteries.
https://www.eneloop.info/home/faq/eneloop.html
https://www.eneloop.info/products/chargers.html

John

 

[audioguru]

I switched to Sanyo Eneloop several years ago because of its low self-discharge rate.
Some competitors are now making similar claims.

New Energizer Ni-MH cells claim Low Self-discharge. It says on them, "Made in Japan for Energizer" so maybe they are Sanyo Eneloop ones.

 

[MikeMl]

You should be thinking about this differently.

You should make your alarm installation such that it is powered from the vehicle battery (not from the AAs) as long as the car battery is alive and well. The only time any current should be drawn from the AAs is if a thief steals the car battery first...

This could be accomplished with just a couple of diodes, no trickle charging of the AAs is necessary. If no current is drawn from the AAs, then you can rely on them keeping their full charge until needed because of their intrinsic low self-discharge rate.

The question about what impact would have on the car battery has to do with what the current required by the alarm while it is armed, waiting for a thief to do something. If that standby current is a few mA or less, then your car battery will run the alarm for a week or two and still have plenty of charge available to crank the starter.

 

[thomasPlam]

hi Nigel & mike

Yes it is permanently connected to the car. There are two reason why i used a AA battery for the unit.

1) The unit should work even if the thief managed to cut my car battery.

2) I don't want to hurt my car battery by directly powering my unit. I will take my car once in a while, so chances of battery getting charge is few.My unit will take only few milli amps current,so it can active for more than one week with a AA battery. Once in a while i need to charge it and hope that wont harm my car battery.

Is their any problem with fast charging, I have setup for preventing battery from overcharged.

thomas

 

[Mikebits]

Design the alarm such that it detects battery compartment intrusion. Use the AA's as backup batt in case main batt fails.

 

[jpanhalt]

The Battery University and the manufacturer sources frown on fast charging NiMH. I charge my eneloop's at 0.8A (<0.5C). As I recall, the instructions warn about charging at more than 0.5C or 1A (I don't recall whether it gives the limit as current or in terms of battery capacity). Heat during charging is the biggest concern.

John

 

[Nigel Goodwin]

hi Nigel & mike

Yes it is permanently connected to the car. There are two reason why i used a AA battery for the unit.

1) The unit should work even if the thief managed to cut my car battery.

2) I don't want to hurt my car battery by directly powering my unit. I will take my car once in a while, so chances of battery getting charge is few.My unit will take only few milli amps current,so it can active for more than one week with a AA battery. Once in a while i need to charge it and hope that wont harm my car battery.

Is their any problem with fast charging, I have setup for preventing battery from overcharged.

No problem, just no point - simply trickle charge it from the car battery, with a single resistor and a reverse blocking diode.

It really couldn't be any simpler or cheaper.

 

[thomasPlam]

Hi john,

Thank you very much for posting quite useful threads. my car battery size is 35 (battery standard size) 80Ah. i don't allow my AA battery to discharge totally.I have some low voltage detect setup, Some trip point i have set around 4.1v when my battery reaches that level, it will start charging from my car battery. when it fully charged i have some high voltage detect also around 4.8v, at this level it will stop charging. For this kind, whether a fast or pulse charging is better.

thomas

 

[jpanhalt]

I agree that intermittent charging is better than continuous trickle charging. By "fast", I am assuming you mean a standard charge rate of ≤0.5C. NiMH do not do well with fast charge rates of several times C. The following applies to NiMH, unless stated otherwise.

I don't know of any evidence that shows a difference in long term performance between pulse charging to keep at full capacity, as described by Panasonic and Battery University, and regular charging when the battery reaches a certain state of discharge. The advantage of pulse charging is that the battery is always at full charge. However, if you set your recharge trigger to occur when, say 60% to 70% of charge remains, your alarm battery should still function well as the alarm and you will reduce the number of recharge cycles. eneloop batteries now carry a claim of 1500 cycles, which is large for NiMH. You can do the calculations from that. Self-discharge cannot be ignored with NiMH batteries, and I suggest you consider the newer, low-self-discharge batteries. I have experience with the Energizer mentioned above and eneloop. There is also a version from China with similar claims, but I have no experience with it.

As for your car battery, don't underestimate the effect of a slow drain on it. Here is a chart from Battery University (**broken link removed**

View attachment 67092

A 50 mA discharge does not sound like much for a large car battery, but that is 1.2 Ah per day. If the car sits for 10 days, that is 12 Ah in addition to the normal self-discharge. That might not hurt an 80-Ah battery, but a smaller 20 Ah to 40 Ah starter battery will likely be damaged.

John

 

[jpanhalt]

Sorry, I missed the second part of your comment when writing my response in #11.

Considering this is being used an alarm, I think your low-voltage trip point is too low. See the charge/discharge curves here: https://www.electro-tech-online.com/custompdfs/2012/09/320Nickel20Metal20Hydride.pdf

For my model airplanes, I would never consider taking off with a voltage per cell of just 1.2V (4.8 total). The discharge curve is just too darn steep at that point.

As for charging, you will find that commercial "peak" chargers generally charge a 4-cell pact to 5.2 to 5.4V or so. Again reference the charge curve. The additional reference in this post gives a good discussion of end-of-charge detection based on a slight drop in voltage.

John

 

[audioguru]

Another problem with stopping the charge at only 4.8V is that one cell might be only 0.9V and is almost dead when the remaining 3 cells are 1.3V each which are also not fully charged. The cell with the lowest voltage will keep getting lower and lower until it gets a reversed voltage during discharge.
All 4 cells must be fully charged to 1.4V to 1.5V each (a total of 5.6V to 6.0V).

 

[Nigel Goodwin]

One resistor, one diode - all that's needed - I can't believe you're discussing fast charging, switching it in and out, detecting low voltages etc.

Which do you think is going to be more reliable for an alarm?.

 

[MikeMl]

All of the energy required to run the alarm ultimately comes out of the car battery!!!!!

Why are you agonizing about the stupid AAs? All they have to do is power the alarm for about 10min if (and only if) the thief cuts the car battery cable...

 

[thomasPlam]

hi Nigel &mike

Guys i am new to this battery management stuffs and little knowledge .. I Asked about all different charging methods for my understanding only.Once i am clear about this may be i will get some better idea .. I am really thankful for your help guys. Sorry for irritating!!

 

[thomasPlam]

hi audioguru,

Another problem with stopping the charge at only 4.8V is that one cell might be only 0.9V and is almost dead when the remaining 3 cells are 1.3V each which are also not fully charged. The cell with the lowest voltage will keep getting lower and lower until it gets a reversed voltage during discharge.
All 4 cells must be fully charged to 1.4V to 1.5V each (a total of 5.6V to 6.0V).

I am going charge the battery in series, so that will eliminate the individual battery voltage imbalance am i right ?

thomas

 

[audioguru]

I am going charge the battery in series, so that will eliminate the individual battery voltage imbalance am i right?

No.
The battery cell with the lowest capacity will be discharged more than the others but in series they all get the same amount of charge. Then if they are not overcharged a little, the weak cell will get less and less of a charge.
Your voltage limit during charging is too low so the cells are never fully charged.

 

[thomasPlam]

hi audioguru,

i got it. thank you very much man