# Low battery indicator

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#### robk

##### Enlightened
I've been playing with a few circuits that will light a red T1 LED mounted in a Mag 2D just above the switch when battery voltage drops to a specified level. My question is : what voltage should I set it to for a 6AA NiMH driven 5W LS? I was thinking 6V for turn-on of the indicator, but maybe I should let the cells run down to .8V each (4.8V). But I don't think I'd be getting any useful light at 4.8V. Any ideas?
Thanks,
Rob

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#### Bullzeyebill

##### Moderator
Staff member
I could of used one of those (low battery ilndicator) the other day when I was doing a run time on my first run Arc LS1. With still plenty of light left, I had driven NiMH's to alsmost 0.80 volts each. Put them back on the charger on low, and the red lights started flashing. Quickly switched to high, and they started charging. That was close!

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#### zmoz

##### Enlightened
I've allways heard that .8v is the minimum for NiMh. Care to share any more details about your circuit? /ubbthreads/images/graemlins/smile.gif

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#### robk

##### Enlightened
zmoz,
the paper work is in the garage, too late to go way out there tonite, but I'll post the circuits tomorrow if I have time. It's simple stuff, but I feel it would be very usefull for DD lights.
Rob

#### shankus

##### Flashlight Enthusiast
I suppose it depends on what you want to define as a "low battery".
Voltage low enough to be dangerous to the cell, or 50% brightness voltage, or something else?

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#### eluminator

##### Flashlight Enthusiast
You are assuming that the voltage of each cell in series will be the same as the cells discharge. That doesn't happen. Before most of the cells have gotten down to 1.0 volts, there will probably be one that goes to 0.0 volts or lower. It all depends on the effective capacity of the cells and that varies. I guess that's because the crystal size varies from time to time.

It seems nobody knows what really goes on inside the cells. Some tell me that once a cell is discharged past zero volts it will be ruined. I have never had a cell ruined and I assume many have been so discharged. It certainly greatly reduces the ability to take a charge, but a couple of charge-discharge cycles seems to fix that.

#### Ray_of_Light

##### Flashlight Enthusiast
The problem is the reverse charge induced from the other good cells in the battery pack...
A NICD, and even worse the NiMH, looses their ability to recharge correctly when they are reverse-charged.
The reverse charge happens when one cell in a series of three or more cells ends its active role: then is being reverse charged from the others.
In a 6 volt NiMH battery pack, there are 5 cells in series, each giving 1.2 volt. When the first cell will end discharging, only 4.8 volts will be available. So, 4.8/5= 0.96 Volt. In practical terms, if you disconnect the load at 5 volts, there is NO possibility of reverse charging.

Hope this helps

Anthony

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#### eluminator

##### Flashlight Enthusiast
I believe two cells in series can result in one going negative if the device drains the cells far enough. My Logitech mouse will discharge both cells to something around .7 volts if the cells are of equal capacity. I often see one of the cells with around .1 volt when I remove them and analyze them. I assume they have been driven negative. The cell that measures only .1 volts or less will sometimes take only a tiny charge the first time. But I've always found that two charge-discharge cycles will get it back to normal. One time the mouse quit working after only one week instead of the usual month and I found one cell with 1.27 volts and the other with .05 volts. After rejuvenation both cells now have the same capacity.

I think when people find the cells don't charge at all it's because they are using a not-so-smart charger. My Quest charger would sometimes refuse to charge but the CCrane has never met a battery it didn't like. I guess it's the "soft start" phase that does the job.