[NLRS] Battery charge regulator

Wed Sep 7 13:06:39 EDT 2011

There are two issues here.

The first is that Gel batteries (and to a lesser extent AGMs) cannot tolerate being over charged and this means a charger that is both voltage and current limited.  In particular, if you actually have a Gel battery, they have a lower max voltage than AGM and flooded so they would be easy to cook with a car charging system.

If the charger is properly current limited, I don't see a problem (except possibly noise/hash from the inverter) from leaving the charger connected while operating.  The charger will supply whatever current it is limited to (which needs to be set at a safe continuous duty level for the charger) and the voltage will fall off until the battery will fill in the rest.

If you have a voltage regulated charger (matched to the battery float voltage), the really simple  (and suboptimal from a power efficiency and longer than necessary charging time) answer, would be to connect the charger through the resistor and the load via a separate connection to the battery positive terminal.

In practice a 1.4 ohm resistor will limit the charging current to much less than 10 amps in a 12 volt system.  While the charger voltage may be near 14 volts (which is too high for a Gel battery, but probably ok for cycle use of an AGM), the battery voltage probably shouldn't be allowed to get below 11 volts, so you would only have about a maximum 2 amps charge rate and that will drop quickly as the battery voltage recovers as it is re-charged.

Jim
KB0UAZ

On Sep 7, 2011, at 11:42 AM, Bill Ockert wrote:

> 
> 
> Hi John,
> 
> Not sure if it is applicable to the overall situation but...
> 
> The trolling motor on my boat uses 24V so has two 12V batteries in series. 
> The boat
> overall therefore has three batteries, on for cranking and two for the 
> trolling motor. This
> is a common situation. I bought a charger from Cabelas that changes all 
> three in place.
> In addition to changing all three batteries from 120VAC, the model that I 
> have will charge
> the two 12V series batteries from the charge current the boat motor puts out 
> to the cranking
> battery.
> 
> I do not recall the make and model of the charger but looking at Cabelas web 
> site there
> are several models that do this.
> 
> Hope this is of some help.
> 
> 73 de Bill ND0B
> ----- Original Message ----- 
> From: <tosca005 at umn.edu>
> To: "NLRS Reflector" <nlrs at mailman.qth.net>
> Sent: Wednesday, September 07, 2011 11:30 AM
> Subject: [NLRS] Battery charge regulator
> 
> 
>> 
>> 
>> Well, I managed to "cook" one of my 31 AH gel cell batteries while trying
>> to recharge it. It didn't explode or rupture spilling acid all over the
>> place, but the case sure looks funny as it pushed outward from internal 
>> gas
>> liberation. Fortunately, it held together without cracking open, but...
>> 
>> Anyway, not wanting to repeat that experience, I am looking for a battery
>> charge regulator. I realize that a 1.4 ohm 150 watt resistor in the
>> positive lead of the (replacement) battery would limit the charge current
>> to about 10 amps, but it would also limit the drain current to the same 10
>> amps or so, which is not enough when roving when some of the "low
>> microwave" amps kick into high gear. I would like a device or circuit that
>> would limit the charging current into the battery to whatever is optimal
>> for a gel cell battery, but not limit how much current is drawn out of the
>> battery under severe load.
>> 
>> An even tougher need is a way to put two 12v gel cells in series to 
>> provide
>> 24 volts at high current for a few of the amplifiers that need 18-26 
>> volts,
>> and still be able to safely charge the pair of batteries from the 
>> vehicle's
>> 12 volt electrical system. I have a 12V to 24V inverter that can deliver
>> about 30 amps at 24 V, but some of the amplifiers want even more than 30
>> amps when driven to the max. I figure that a pair of healthy gel cells in
>> series could provide all the current I need for short periods of time, as
>> long as I can keep them properly charged. Using the 12-to-24V inverter
>> output across the series-connected 12V batteries would potentially work, 
>> if
>> not for the problem of lack of charging current regulation leading to
>> another catastrophic battery failure, which would not be nice on the road.
>> 
>> Ideas, anyone?
>> 
>> 73 de W0JT
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> 
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