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ZL1ANM > HELP 24.03.05 03:41l 61 Lines 2856 Bytes #999 (0) @ WW
BID : ZL1ANM400511
Read: GUEST
Subj: Re: Cheap n nasty gel cell chgr ?
Path: DB0FHN<DB0NOE<DB0GAP<DB0FSG<DB0MRW<DB0ERF<DB0FBB<DB0BI<DB0NOS<DB0EA<
DB0ACC<DB0GOS<ON0AR<ON0AR<ZL2BAU<ZL1AB
Sent: 050324/0236Z @:ZL1AB.#11.NZL.OC #:13214 [AUCKLAND] FBB7.00g25 $:ZL1ANM400
From: ZL1ANM@ZL1AB.#11.NZL.OC
To : HELP@WW
>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.
>
>73 de Andy GM7HUD
The advice given above is just plain downright bad, and runs contrary
to all manufacturers' recommendations about current limiting. It is
also unsafe advice: a battery may have an internal short circuit.
The best way I've found to charge a 12v, 7 AH gel cell battery is to
connect it to a 13.5 to 13.8 volt power supply via a 3.3 ohm, 5 watt
wirewound resistor. Measuring the voltage (2v or less) across the
resistor and multiplying by 300 gives the charge current in mA.
The resistor limits the initial charge current to less than 500 mA
(depends on how long ago the battery was disconnected from the load
because its voltage rises after disconnection: recent disconnection =
higher current, standing idle for a while = lower current).
While charging, the battery voltage rises and eventually reaches the
voltage of the power supply. At this point, it is not yet fully
charged, so the power supply voltage may be raised to the 14.4 volts
recommended by most manufacturers. The battery voltage will then rise
fairly quickly to 14.4 volts and charging should be stopped promptly
at that point. Serious loss of battery performance will result if
charging is prolonged at that level.
The reason for initially charging at 13.5 to 13.8 is that cells do
not accept charge at the same rate. There is a tendency for the
fast cells to be overcharged because the slower ones are dictating
a rate of charge which is too high for them. So a lower terminal
voltage for the first stage of charging is far safer than charging
at 14.4 volts from the start.
When charging is completed and you don't need to use the battery
straight away, you can maintain its charge by disconnecting it,
allowing its voltage to drop to about 13.6v, set your power supply
to 13.6 and float charge the battery at that level until its needed.
The resistor should be in circuit at all times, and in addition to
limiting the initial switch-on current, it serves as some protection
against catastrophe should the battery develop an internal short.
73 de Neil ZL1ANM
in Auckland.
T4 1.5à24
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