[NLRS] Battery charge regulator

[Doug Reed]

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.

KBØNLY wrote:
> <snip>  get a couple Optima Yellowtop deep cycle
> batteries, these are 55Ah each and can handle being charged by an automotive
> alternator without any dropping resistor or additional regulation.
> <snip>
> You could run a pair of them for 24v, I did this for field day once to run
> an amp, voltage select you could do it simple enough with a relay setup to
> latch the batteries together for 24v and split them for 12v and ease of
> charging while going down the road.  I did this with a pair of 40a DPDT
> relays, you could get by with a few SPDT relays if you needed more current
> capacity, such as continuous duty rated contactors, I just used what I had
> on hand.  The first relay gets the positive from one battery to the common
> of one of the relays poles, the NC contact for that common goes to the
> positive of the second battery, the NO contact goes to the negative of the
> second battery.  The second pole of the relay has its common to the negative
> of the second battery and the NC contact to the negative on the first
> battery and the combined ground terminal on the bus strip for both 12v and
> 24v negative.  The 12v positive comes off the first battery, so the 12v
> devices are always powered off the first battery even when the 24v selection
> is made.  The 24 positive comes off the second batterie positive which goes
> to the common of the second relay, the NO contact becomes the 24v output.  I
> wired this all up with a simple toggle switch to energize the relays and a
> terminal strip with connections for both voltages making a bus strip.  I can
> draw this up if someone wanted to see a picture.
>
> The way this works is you connect your 12v charging input from the vehicle
> to the 12v terminal on the bus strip and the combined ground terminal, with
> the relays off both batteries are charged by the vehicle and all 12v devices
> have power from both batteries being in parallel, when you flip the switch
> and turn on the relays the 12v devices and charging input is still connected
> to the first battery but now you also have a 24v output with both batteries
> in series.
>
> 73,
> Scott KBØNLY