[Elecraft] GellCell Charging with bulb

[Don Brown]

Hi

Maybe I can help. Gel cells should not be allowed to discharge below 10 
volts and charged above 14.5 volts or damage may result. If you want to 
maintain a float charge on the battery you should set the power supply to 
13.8 volts and limit the current to about 1/3 of the rated amp hour of the 
battery. You can do this with a current limiting power supply or you can use 
a resistor in series with the battery to limit the current. When the battery 
is fully charged the current draw at 13.8 volts will be only a few mA. If 
the battery is discharged then the current can be quiet high without some 
form of current limit. The difference between 13.8 volts and say 11 volts in 
a discharged state is 2.8 volts. So in the worst case you need to calculate 
the the resistor value that would limit the current to 1/3 of the amp hour 
rating of the battery. In the case of a 7 amp hour battery that is 2.33 
amps. 2.8/2.33 is 1.2 ohms. I have some 4 watt 12 volt lamps used in low 
voltage outdoor lighting that read 3.4 ohms cold this should work for you or 
a dash lamp for a car or even a car tail light as long as it is 1.2 ohms or 
higher. Do not set the power supply higher than 13.8 volts even with the 
limiting resistor (bulb) as the battery will continue to charge to whatever 
the power supply is set to.

Don Brown

KD5NDB




----- Original Message ----- 
From: "Thom R LaCosta" <lacosta at bcpl.net>
To: "Elecraft email" <elecraft at mailman.qth.net>
Sent: Wednesday, November 01, 2006 4:40 PM
Subject: [Elecraft] GellCell Charging with bulb


> Realizing certain pitfalls, can someone tell me what typle of bulb I can 
> put in
> series between a dc source and a 7 ah gell cell to maintain a charge on 
> it...I
> have more gell cells than proper chargers, and my thought was to get them 
> up to
> full charge with a "real" charger and then use the series bulb to maintain 
> the
> charge.
>
> Thanks
>
> 73,Thom-k3hrn
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