Want an "emergency battery"?

David Beale · 03-10-2007, 12:19 PM

Costco sells NIMH batteries, 6 AA 2500 mAHr and two AAA 900 mAHr in a blister pack (Sanyo cells). I bought two (for another project). Turned out I didn't need the AA cells, so I decided to replace the gel cell in my house alarm with them. Mouser sells some really nice cell holders made by Keystone. They're aluminum, and very well made. I purchased a bunch and made myself a 10 cell battery (which is what you want for a 12 V system). I used 16 gauge wire to hook them up to keep losses down, in case I wanted to draw some big current from them.

Now, it occured to me this battery would do fine to "jump start" the Prius, should your 12 V gel cell go dead on you. You could even keep it in the car, trickle charging from an acc. outlet when you drive. I'd run that through a 100 ohm resistor just to be sure you don't charge it too heavily. You can then just clip it to the terminals under the hood to "jump" the Prius if the gel cell dies on you (not through the resistor, direct from the battery).

It gives me a chuckle to think such a tiny battery can jump a car.

kevinwhite · 03-10-2007, 12:36 PM

<div class='quotetop'>QUOTE(David Beale @ Mar 10 2007, 09:19 AM) [snapback]403397[/snapback]</div>

Quote:

Costco sells NIMH batteries, 6 AA 2500 mAHr and two AAA 900 mAHr in a blister pack (Sanyo cells). I bought two (for another project). Turned out I didn't need the AA cells, so I decided to replace the gel cell in my house alarm with them. Mouser sells some really nice cell holders made by Keystone. They're aluminum, and very well made. I purchased a bunch and made myself a 10 cell battery (which is what you want for a 12 V system). I used 16 gauge wire to hook them up to keep losses down, in case I wanted to draw some big current from them.

Now, it occured to me this battery would do fine to "jump start" the Prius, should your 12 V gel cell go dead on you. You could even keep it in the car, trickle charging from an acc. outlet when you drive. I'd run that through a 100 ohm resistor just to be sure you don't charge it too heavily. You can then just clip it to the terminals under the hood to "jump" the Prius if the gel cell dies on you (not through the resistor, direct from the battery).

It gives me a chuckle to think such a tiny battery can jump a car.

[/b]

It is not Prius related but if you don't mind me asking - why are you replacing the gel-cell in your alarm with NIMH cells?

In general NIMH cells are not suitable for float-charging as is used in a house alarm. I would expect the batteries to fail within a few months, although you may not notice it until you have an AC power failure.

Lead Acid (including gell cells) batteries work well when they are being float charged (and have done for over 100 years in telephone and railroad service) because the charge current drops to a very low level when they are charged at a constant voltage - this doesn't occur with NiCd or NiMH types and the electrolyte will gradually evaporate.

kevin

David Beale · 03-10-2007, 06:23 PM

Actually, if you use a low "trickle charge current", NIMH and NICad batteries work very well. I have a digital alarm clock that uses them, which I built in 1975. I have had to change the batteries once so far. The trick is to make sure they don't heat up from the trickle charge. The emergency battery charge cct above would trickle charge that battery with about 30 mA. That wouldn't heat up a 2500 mAHr battery enough to cause any problems.

A gel cell would be expected to last between 5 and 8 years under trickle charging (they fail due to sulphation even when fully charged once they reach that age area).

Bonus, if there is some kind of charge failure, there is no damage to a NIMH battery if it fully discharges (as long as you are not drawing current from it). Allowing a gel cell to fully discharge will destroy it in short order.

The charge cct I mentioned above can be made "fool proof" by putting the resistor across a 6 Amp diode. The diode cathode is the output (to enable the car to start) and the input for charging. That way, if you left the battery connected for too long to the car once it was running you would not overcharge the little AA battery.

tochatihu · 03-11-2007, 12:19 PM

Not sure that it has been established yet that this small size of NiMH battery has sufficiently low internal resistance for prius bootup. Cordless tool battery packs (presumably NiCd) have been used, so it may be a fair bet.

On the other hand, alkaline D cells in series were inadequate when I tried them.

PbA gel cells of size 2 amp hours and larger certainly work for this.

03-10-2007, 12:19 PM	#1
David Beale Senior Member Join Date: Jul 2006 Location: Edmonton Alberta Posts: 1,556 My Car: 2007 Prius Package: B Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Costco sells NIMH batteries, 6 AA 2500 mAHr and two AAA 900 mAHr in a blister pack (Sanyo cells). I bought two (for another project). Turned out I didn't need the AA cells, so I decided to replace the gel cell in my house alarm with them. Mouser sells some really nice cell holders made by Keystone. They're aluminum, and very well made. I purchased a bunch and made myself a 10 cell battery (which is what you want for a 12 V system). I used 16 gauge wire to hook them up to keep losses down, in case I wanted to draw some big current from them. Now, it occured to me this battery would do fine to "jump start" the Prius, should your 12 V gel cell go dead on you. You could even keep it in the car, trickle charging from an acc. outlet when you drive. I'd run that through a 100 ohm resistor just to be sure you don't charge it too heavily. You can then just clip it to the terminals under the hood to "jump" the Prius if the gel cell dies on you (not through the resistor, direct from the battery). It gives me a chuckle to think such a tiny battery can jump a car. __________________ Edmonton Alberta "Pearl" is a 2007 Driftwood Pearl Prius.

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03-10-2007, 06:23 PM	#3
David Beale Senior Member Join Date: Jul 2006 Location: Edmonton Alberta Posts: 1,556 My Car: 2007 Prius Package: B Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Actually, if you use a low "trickle charge current", NIMH and NICad batteries work very well. I have a digital alarm clock that uses them, which I built in 1975. I have had to change the batteries once so far. The trick is to make sure they don't heat up from the trickle charge. The emergency battery charge cct above would trickle charge that battery with about 30 mA. That wouldn't heat up a 2500 mAHr battery enough to cause any problems. A gel cell would be expected to last between 5 and 8 years under trickle charging (they fail due to sulphation even when fully charged once they reach that age area). Bonus, if there is some kind of charge failure, there is no damage to a NIMH battery if it fully discharges (as long as you are not drawing current from it). Allowing a gel cell to fully discharge will destroy it in short order. The charge cct I mentioned above can be made "fool proof" by putting the resistor across a 6 Amp diode. The diode cathode is the output (to enable the car to start) and the input for charging. That way, if you left the battery connected for too long to the car once it was running you would not overcharge the little AA battery.

03-11-2007, 12:19 PM	#4
tochatihu Senior Member Join Date: Apr 2004 Location: Kunming Yunnan China Posts: 1,823 My Car: 2001 Prius Package: Pioneer #1 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 7	Not sure that it has been established yet that this small size of NiMH battery has sufficiently low internal resistance for prius bootup. Cordless tool battery packs (presumably NiCd) have been used, so it may be a fair bet. On the other hand, alkaline D cells in series were inadequate when I tried them. PbA gel cells of size 2 amp hours and larger certainly work for this.