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[OM] Re: Metz 60 CT-4 Dryfit Battery Rejuvenation

Subject: [OM] Re: Metz 60 CT-4 Dryfit Battery Rejuvenation
From: Tim Hughes <timhughes@xxxxxxxx>
Date: Fri, 25 Jun 2004 02:43:09 -0700 (PDT)
John wrote:
>>
A fellow list member, and I've been unable to find who it was after 
scouring my email archives, gave a number of suggestions, including deep 
cycling it several times and simply leaving it on trickle charge for a very 
long time. 
<<
Likely me, I have designed high performance chargers for Sealed Lead Acid 
batteries professionally, and have cycled them to life and death many times, 
while optimizing chargers.  Generally SLA's are used in mainly inexpensive 
equipment so the chargers are often poorly designed to save cost.   The battery 
quality/price/ performance varies a lot between vendors.  With care they can be 
rapidly charged and achieve better cycle performance than even the most 
optimistic claims of the vendor. For example I once cycled (~full discharge) 
some Hawker brand (Cyclon) batteries to 1500 cycles (4 cycles/day ~14 months = 
over 2000 Watt hrs!). Near the end, they started to loose capacity fast as 
parts of the plates internally were starting to disconnect. The vendor's claims 
under their recommended charge regime was 200cycles at that time.  The most I 
ever did for the Sonnenschein batteries (like are used in Metz) is 700cycles, 
but I stopped testing (the capacity was still over 85%). The pris
 matic
 (flat)sonnenschein plates likely wear out earlier than Hawker (rolled).
 
There are actually a number of different SLA technologies. The Sonnenschein is 
termed an excess electrolyte design, using fumed silica in the gelled 
electrolyte to enhance gas recombination. It has relatively good sulphation 
recovery, as long as you don't ever leave it deep discharged for a really long 
time (>30days). Charge the sooner the better and float charge in the Metz 
charger for days
 
To recover the batteries it helps to cycle them with a charger having a little 
higher regulated voltage supply than that used in the Metz charger cct (which 
is actually in the 60CT box, not the wall wart) .  The voltage used by Metz 
(2.33V/cell ) is really  more appropriate for float use than cycle use.  It 
would be better to use a smarter charger and 2.5V/cell (i.e. 7.5V for Metz 6V 
battery) or use a current source capable of 5-10mA to top up the cells after 
normal charge. If you use the higher voltage, you need to be careful not to 
leave it on charge too long or it may thermally run away. Use a timer or put an 
outdoor thermometer on them and keep an eye out!  You can make a crude current 
charger for topping up batteries using a 12V wall wart, and hook up a resistor 
to limit current. (~560 Ohms, ~8mA). You can comfortably leave it on top up 
current for a few days without a problem when recovering. 
 
Regards,
Tim Hughes


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