[NLRS] Battery charge regulator

[Dr. Gerald N. Johnson]

Lead acid battery fundamentals.

The optimum charging voltage is sensitive to the battery, primarily the 
electrolyte, temperature. Ambient temperature may approximate the 
battery temperature, but the battery with lead and sulfuric acid 
electrolyte has a large thermal mass. Far greater than air. The battery 
is warmed by currents entering or leaving.

The optimum charging voltage is sensitive to the electrolyte full 
charged specific gravity selected at the factory.

What I call the optimum charging voltage is the maximum voltage where 
the charging current tapers to zero which gives the maximum stored 
energy. A slightly lower float charging voltage can be used at the cost 
of some stored energy.

Overcharge energy is usually dissipated in dissociating the water of the 
electrolyte into hydrogen and oxygen. The bubbles look like boiling but 
aren't hot. These gases come out at the optimum ratio for the best bang 
if ignited, two parts H to one part O. Though H will burn in air at 
concentrations over the range of 4% to over 80%. Venting is a good idea 
for enclosed batteries.

The choice of two 6 volt batteries in series vs 12 volt batteries in 
parallel comes up on New Ag Talk Machinery about once a month as applied 
to starting diesel batteries. Golf cart batteries are not considered for 
those tough starting situations. Tractor manufactures have used both 
schemes. Today, for starting batteries, opinions lean towards a super 
sized 12 volt or two 12 volt in parallel, primarily because the 
available 6 volt are getting to be uncommon, and made without some of 
the later constructions that reduce voltage drop between cells. Either 
scheme has its benefits and its faults.

The ampere hour rating of two 6 volt batteries in series is that of the 
weakest battery of the pair. The ampere hour rating of two 12 volt 
batteries in parallel is about the sum of the individual ratings. An 
open cell in the 6 volt batteries in series can prevent supplying any 
loads. An open cell in one of the 12 volts in parallel reduces the load 
capacity but it stays above zero.

Battery selection for two 6 in series is less critical, one with a 
different SG will still charge, though they need to have identical 
amperehour capacities to charge and discharge equally. Paralleled 
batteries need to have the same full charge SG and be in the same 
condition. A shorted cell in one battery will discharge the other 
battery. If the specific gravity is not the same they won't charge the 
same or share the loads well. Its important for the paralleled batteries 
to be replaced together from the same manufacturing lot.

By the way I've found over the decades that learning to read the 
manufacturing date code and digging to the back of the pile to get the 
most recently made batteries gives me a much better battery life than 
taking the oldest one that the seller would prefer I take. Sitting 
uncharged for months takes much of the life out of a new battery. 
Fortunately for ordinary starting purposes they are way over sized when 
new so that loss of life isn't noticeable fore a couple years. In some 
stores I can get to the back of the battery rack, but in others I can't 
and I don't buy batteries there unless the rack was empty the day before 
and I can see the current month on the battery date identification.

The series combination of batteries puts more connections to go bad in 
series with the load but those connections are cleanable and need 
regular attention. Eons ago the battery posts were cast in place and 
filled the opening but today the posts and the tops are cast separately 
and there is enough gap to let fumes through and sometimes those fumes 
include sulfuric acid which eats at the connections. Enough on my truck 
that a couple months ago, one of the brass battery post clamps split in two.

I think today you will find that heavy duty 12 volt batteries will 
deliver more watts (or amperehours) per dollar invested than 6 volt 
batteries because the market is bigger. Good golf cart batteries may 
prove me wrong on occasion.

Farm and industrial tractors as well as semi tractors have been made 
with 12 volt run and charging and 24 volt starting so the switching 
relays do exist, though if made for cranking a 300 hp diesel engine the 
relay will be a bit overkill for a mere 1 KW power amp. Some late 50s 
and 60s Deere diesel tractors (before 1969) used a dual voltage system. 
Two 12 volt batteries in series with the mid point grounded. Light and 
accessory loads were split between the positive and negative 12 volt 
circuits, while the charging generator and the starter floated running 
on 24 volts. Very much like an early Edison DC power system at 1/10th 
the voltage. Its workable, though it confuses the neophyte trying to 
service it and its popular to convert to simple 12 volts like later 
tractors where Deere sometimes used two 6 volt in series or two 12 volt 
in parallel. The main failure of that dual voltage system is that the 
charging generator was way too small and it took a half a day to make up 
for a tough start and by then the fuel tank was likely empty. The next 
failure came from brush dust grounding one of the brush holders in the 
starter upsetting one battery's charging. Users upset it by adding 
accessory lamps and radios without balancing the loads. And the 24 volt 
loads were not grounded which won't work for power amplifiers.

While battery switching is practical, the modern boost switching 
regulator should also be considered for getting 24 volts from 12. They 
are available as isolated 12 to 12 modules where the output can be put 
in series with the existing 12 volt supply.

73, Jerry, K0CQ

On 9/9/2011 11:09 AM, Doug Reed wrote:
>
>
> I like Scott's relay trick the best. If you wanted to get real tricky
> you could probably put the relay control into the PTT sequence and only
> switch them to series during actual transmit, if the amps can turn on
> fast enough.
>
> For the cost of a big relay, this proposal is certainly the easiest to
> do. It does assume the vehicle charge system will not destroy the
> batteries, but during the course of a two day contest it is unlikely to
> do that. I'd be particularly sure that things are well protected in case
> a relay switches late or a contact fuses and shorts a battery.
>
> I suppose if I was going to extend Scott's suggestion, I'd use a
> DC-to-DC "universal" laptop power supply to provide 22-24 volts through
> a diode to provide standby power to the amp and also for driving relays
> if needed. Then PTT switch the batteries to a series configuration
> during transmit to supply the high current. All charging would be done
> by the vehicle alternator and charging system.
>
> As for long term battery float charge operation at a fixed site, I tend
> toward isolating the battery from the station supply with a diode and
> use a continuous duty AC relay so the contacts short the diode when AC
> power fails. Or use the AC relay to switch a high current 12V relay to
> short the diode, assuming you can spare the extra current. If you don't
> want the AC power supply across the DC bus when AC is off, you can use
> two diodes and have the relay contacts short the appropriate diode
> depending on external AC. Use a cheap plug-in wall-wart power supply to
> drive the power fail relay. The diodes eliminate the power glitch when
> switching from AC to battery power. Shottkey diodes are best since they
> have the lowest forward voltage drop.
>
> You can use a low current float charger on the battery continuously or
> connect your high-current charger to the battery and put it on a 24 hour
> timer rigged to turn it on for 30 minutes or an hour once a day. That
> will keep the battery charged without over charging. After a power
> failure or when you need to draw down the batteries, just flip the timer
> on and it will automatically shut off again in case you forget. With
> this arrangement the base station power supply is NOT charging the
> batteries.
>
> The 24 hour timer method was used with a cheap battery charger on a
> emergency services truck for many years and kept the batteries in great
> shape.
>
> In the next vehicle we just hung the batteries across an Astron power
> supply and adjusted the supply for the specified battery float voltage,
> leaving the power supplies on all the time. This tended to boil the
> batteries, 13.6 volt float voltage may have been too high. When we did
> finally drain the batteries, the power supply smoked when it tried to
> charge the batteries at max current. (Astron supplies do not current
> limit gracefully......)
>
> To get around that problem, we tried using a smaller power supply with a
> charge current limiting resistor in series to protect against shorts.
> This kept the batteries up but they still tended to loose water and took
> a long time to charge. We finally had a long Skywarn net and drained the
> batteries because the power supply couldn't charge them fast enough
> through the resistor. Instead of a fixed resistor, something like a car
> headlight would be a better option and still limit the charge rate.
>
> We now use an Iota 55Amp power supply and IQ4 module on the batteries in
> the vehicle. I think the batteries still loose water but not as bad. Of
> course the batteries are getting old now and that may be part of the
> problem. I bought an Iota DLS-55 55Amp supply to use in my home station
> too, a good value for the money. If the IQ4 module switches between
> float and charge like Scott said, that gives the best features of float
> charging and cycle charging, kind of like the timer and charger option does.
>
> Most lead-acid rechargeable batteries have two charge ratings, Standby
> (float) use and Cycle use. Standby is when you hook it up to a charger
> and leave it there until needed. Cycle is where you hook it up to charge
> and disconnect it to use when charging is complete. The 6V gel cell
> battery I'm looking at is labeled "Standby use 6.75-6.90V at 20 degrees
> C" and "Cycle use 7.20-7.50V at 20 degrees C." For Standby use I would
> hook it up to a 6.8 volt power supply and leave it. For Cycle use I
> would use a 7.3 volt power supply and disconnect when done. The voltages
> are double for a 12V gel cell battery. Follow the specs on the battery
> or on the manufacturer spec sheet.
>
> There is also a charge rate limit on most gel cell batteries, typically
> 25% of the amp-hour rating. A 4Ahr battery should be charged at 1AMP or
> less, giving a full charge in about 4 hours or longer. For safety and
> battery life, 10% is a better rate. These ratings are usually expressed
> as C/4 or C/10 where C is the amp-hour rating of the battery.
>
> A smart battery charger will adjust the battery charge voltage depending
> on the ambient temperature. A smarter charger will occasionally switch
> from float to cycle because a battery may not get 100% charged when
> sitting at the Standby (float) voltage.
>
> The simplest battery charger and backup switching that I've seen was the
> backup circuit used in several control panels we made at work. It
> consisted of a Schottkey diode to isolate the battery from the power
> supply, and a PTC fuse in parallel with the diode to set the max charge
> rate. If power failed, the diode allowed power to flow from the battery
> with one diode drop max. The PTC fuse in parallel with the diode also
> reduced the diode drop under power fail conditions. When AC was on, the
> PTC fuse connected the battery to the power bus for float charging. If
> the battery was drained, the PTC fuse would open to hold the charge
> current to a rate the power supply could safely handle. We used a 200ma
> PTC fuse to limit charge current to 2 amps or less.
>
> I used this arrangement on some remote battery-backed APRS equipment.
> I'm pleased to say the PTC fuse protected the power supply when the
> battery failed.
>
> One note about PTC fuses. The nominal rating on the device is the CARRY
> current. The "fuse" will open at a significantly higher current
> depending on ambient temperature and duration of the current draw. Look
> at the spec sheet curves for current versus response time. To get down
> under a few seconds response time you have to be nearly 10x the rated
> current to open the fuse. A 2Amp PTC will allow nearly 20 Amps draw for
> a few seconds before it goes "open". Once "open" it will stay that way
> and limit the short circuit current to something just over its rated
> current. If temps are below zero, the response time is longer and
> current is higher since the PTC device has to heat up to work.
>
> I hope all of this helps somebody. I just had to comment because I liked
> Scott's method so much. I just wish the AGM batteries were cheaper....
>
> I'd like to hear from anyone with practical experience using paralleled
> batteries to increase amp-hour ratings. So far I don't trust that
> option. I'd prefer to start with higher amp-hour batteries in series to
> get the ratings needed. The 6V 220AHr golf cart batteries really seem to
> have an excellent price point and I'd trust two of them in series more
> than putting two 12V 110AHr batteries in parallel. But I'd like to hear
> from anyone with real-life experience in the field.
>
> 73, Doug Reed, N0NAS.
>
>