From: Patrick Jankowiak (recycler@swbell.net)
Date: Thu Feb 16 2006 - 20:09:11 PST
For those using two 12V batteries in series, who wish to avoid overcharging
one and undercharging the other, especially when operating some 12VDC
equipment from the midpoint, there have been available various solid state
voltage equalizers. These are commonly used in tour buses and motor homes
to serve large 12VDC loads.
These somewhat large "black boxes" connect to +24, +12, and ground, right
at the batteries. They work by sensing the 24V voltage (27.5VDC for you
purists, nomenclaturists, and terminologists) and comparing it to the DC
voltage at the +12V midpoint of your stack of two batteries.
When the 12V point is less than half of the voltage at the 24V point (often
the case when running that 300 watt CB linear, eh?), the device operates a
switching power supply that taketh from the upper battery and giveth to the
lower battery, so that all things are equal and harmonious. I have one that
can provide up to 50 amps of "make up" current.
Very nice!
However, when ye olde truque sits for a month or so, and the lower battery
should by some chance have less voltage than the upper battery, the device
will use the upper battery to charge the lower battery, keeping all things
equal and harmonious.
I cannot probably explain the reasons why, but this can lead to the upper
battery becoming discharged and the lower battery remaining fully charged.
The truck will not start. It's more likely and the result is worse when the
batteries are not maintained well, such as having low water etc.
Both will batteries read voltage-wise to be approximately OK on a regular
volt-meter. A "battery charge tester" such as you may use to load and check
the charging system and cold cranking amps available from a 12V car battery
will read that the top battery is badly discharged and the bottom battery
is completely full.
This is because: In between the equalizer's 'charge cycles' the top battery
regains a surface charge. The voltmeter on the top battery reads OK, but
when a load is applied, down goes the voltage! (The equalizer operates
continuously under normal conditions but will stop and 'cycle' if the top
battery gets discharged)
The unit supposedly draws 17mA on standby, but at this rate, it would take
122 days to discharge a 'large' 50 amp-hour car battery. -so it must be
actively working.
Putting a 12V battery charger on the top battery will charge it back up and
then you can start the truck. Once the truck is started, the batteries
charge normally. The top battery should be fully recharged before using the
truck's system to charge the batteries so that the series connection does
not cause an overcharge of the already full lower battery.
(My equalizer, a Vanner model 60-50, does not bypass current around the
lower battery nor use the lower one to charge the upper one; the assumption
of its design is that the lower one is the only one to need boosting
because of 12VDC loads draining it)
I will solve the problem by adding a heavy duty double pole contactor so
that the 12V and 24V lines from the equalizer are connected to the
batteries only when the truck power switch is on.
There's a nice PDF manual with theory of operation and FAQ here:
http://www.vanner.com/pdf/man-0001.pdf
I hope this enlightening little experience of mine will help someone else.
Patrick
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