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GM7HUD > HELP 23.03.05 13:17l 55 Lines 2705 Bytes #999 (0) @ WW
BID : D42270GM7HUD
Read: GUEST
Subj: Re: Cheap n nasty gel cell chgr ?
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DK0WUE<7M3TJZ<KP4IG<ZL2TZE<GB7PZT<GB7MAX<
GB7ESX
Sent: 050323/1057z @:GB7ESX.#31.GBR.EU #:26862 $:D42270GM7HUD [Witham, Esx]NNA
M1BYT wrote:-
> GM7HUD Opined with considerable skill:-
>>> If you want cheap and dirty.... 13.8v power supply, with a car headlamp
>>> bulb
>>> in series to restrict the charge current.
>>
>> Wrong!
>
> Why?
>
> It is cheap, exactly as requested. The headlamp bulb will restrict the
>charge
> to at most 2amps or so. I appreciate it will not achieve a full recharge,
> because to achieve that would require 14.4v.
Because Lead Acid batteries need charging from a CONSTANT VOLTAGE supply
for maximum durability. They do not need the charging current limiting. If
you do that you will introduce slope charging which is the quickest way to
reduce their life expectancy after overcharging at high temperature.
The current drawn by a lead acid battery when 1st connected to a PSU
consists of a high peak which rapidly drops to a much lower current. There
is no need to limit the current ever. Large batteries may have an initial
current that is in excess of the PSU capability, this is quite likely
to be very large for the 24Ahr battery that was mentioned.
eg I have a number of 12V 7Ahr units. From a heavily discharged state the
initial current drawn when connected to a 13.8V PSU exceeds 5A but within a
minute that is down to well under 750mA. From then on the current drops as
the battery charges until the terminal voltage essentially matches the
charge voltage and little current flows. Of course 13.8V doesnt fully
recharge the battery, but only charges to about IIRC, 85% capacity. To
fully charge you need 14.4V, but at that voltage it is essential to monitor
the ambient temperature and battery temperature. Charging at 14.4V @
30Celsius will reduce a battery's life expectancy by upto 50% !
A final point is that most of the surplus 13V 24Ahr cells in the UK come
from electric wheelchair units. By the time they hit the amateur surplus
market they are well and truly knackered having been overdischarged in and
badly recharged in service. Unless you know the battery on offer is new,
they are worth hardly anything, only their scrap lead value, as you have no
way of knowing if the capacity is 24Ahr or 1Ahr! Although the chance
remains of finding one that hasnt seen too much service, that works very
well and you get a bargain. A bit like buying untested hard disks that are
dumped on the floor at a flea market :-)
Yuasa, Hawker and Gates produce excellent information on lead acid
batteries and it is well worth seeking out. In addition, as I said before,
the UC3906 lead acid charge controller is avilable from Texas Instruments,
with a few components and some pass transistors provides the dog's doodahs
in chargers including temperature compensation etc.
73 de Andy GM7HUD
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