Battery Charger Amperage Settings

jimtrs

Member
I have a wheeled Schumaker model SE-4020 manual battery charger I purchased new in 2009. It has a selector switch with the following choices:

6 Volt 40 amp / 100 amp start
12 volt maintainer
12 volt 10 amp
12 volt 40 amp
12 volt 200 amp start

Even though this is a manual charger, the amount of current (by the amp meter on the charger) drawn by the battery typically tapers down as the battery becomes charged. I know that even though the draw does taper down, it does not stop charging and can damage a battery if overcharging is allowed. Just as charging at too high of an amp rate can damaged a battery quicker. I believe this has to do with heat.

So if I was to charge a 12 volt battery at the 40 amp setting, when that battery was fully charged, the amp meter would shows that the battery is still drawing approximately half that in amps (20 amps). Same thing if charging at the 10 amp setting. When fully charged the amp meter would show about half of that draw (5 amps).

Now with all that said, I do not end my charging based on that. I have learned how to properly determine a battery charge state, and how to determine charging time at a given amp charge rate. I typically charge 12 volt batteries at the 10 amp charge rate, so as to not overheat the battery with a higher setting.

My questions comes when I am charging a 6 volt battery with this charger. Since this charger has only one 6 volt setting (6 Volt 40 amp / 100 amp start), I would like to know of a way to reduce this amperage down to about a 10 amp setting. Even though the 40 amp setting does taper down, and the 6 volt battery does not draw all that amperage the whole time, the cables and in some cases the batteries themselves start getting hot. Now I know a battery getting hot while charging is not healthy for the battery. Does anyone know of a way to reduce this 40 amp, 6 volt setting down to 10 amps?
 
Farmer fix. Take a 100 watt light bulb and
solder a wire to the center contact and
another to the shell. Run the 6 volts
through the bulb. Take an amp meter and see
how much is going through. The bulb will act
as a current limiter. Lower watt bulbs will
allow less current. It should only be a few
watts, just righr for slow charge. The other
thing is go buy a Schumacher 6-12 charger
maintainer. Everything is automatic.
 
So what do you think the "current" setting actually does? It changes the VOLTAGE! On most chargers, changing the current setting changes which transformer taps are used.

Note that since the battery charger output is not regulated, it's not truly DC but rather full-wave rectified AC. Which is why the current drops as the battery charges: The diodes only conduct when they're forward-biased, so as the battery voltage rises the percentage of time current is flowing through the diodes drops.

So... if you put a resistor between the charger and battery, you'll reduce the charge RATE, but you won't prevent over-charging. Because the peak voltage of the full-wave rectified output doesn't change.

There is a simple way to cut the charge rate in half without using any sort of resistor or load: Cut the wire to one of the diodes and put a switch it in. That will change the output from full-wave to half-wave rectified AC. The switch needs to be able to handle the full 200 amp output of the charger, so you'll need to use a heavy duty battery switch. And only turn off the switched diode on the lowest setting; otherwise you're likely to overheat the other diode. And again, it won't prevent overcharging; it will just take twice as long for it to happen.
 
If the charger uses 2 diodes, remove the wire going
to one. That should cut the amps in half.

I have an old manual charger. I've damaged many
batteries with it. So I put it on a timer and only
use it for an hour or two at a time.

Overcharging, overheating a battery is bad news for
a battery.

I have a variac, variable transformer. I can reduce
the input voltage to charger, which will reduce the
output current. You can turn a manual charger into a
trickle charger that way.

The price of a good smart charger is less than the
cost of a good 6v battery. Bite the bullet, but
another charger.
 
(quoted from post at 08:56:19 04/23/17) The price of a good smart charger is less than the
cost of a good 6v battery. Bite the bullet, but
another charger.

I have considered this, but have had mixed results with automatic chargers. Not charging weak batteries that I have been able to bring back manually. Not charging batteries to full capacity.

Maybe due to the fact that the automatic chargers I have used were ones given to me and of questionable quality.

Anyone got good experiences with automatic chargers?

(quoted from post at 07:18:44 04/23/17) So what do you think the "current" setting actually does? It changes the VOLTAGE! On most chargers, changing the current setting changes which transformer taps are used.

I didn't really know how the selector switch reduced the amps the battery could draw.

Note that since the battery charger output is not regulated, it's not truly DC but rather full-wave rectified AC. Which is why the current drops as the battery charges: The diodes only conduct when they're forward-biased, so as the battery voltage rises the percentage of time current is flowing through the diodes drops.

I see

So... if you put a resistor between the charger and battery, you'll reduce the charge RATE, but you won't prevent over-charging. Because the peak voltage of the full-wave rectified output doesn't change.
I understand about overcharging. The battery charger has a timer, which I monitor. As I said in my earlier post I can determine the correct amount of charge time at a given amp rate. I monitor this, and check it.

There is a simple way to cut the charge rate in half without using any sort of resistor or load: Cut the wire to one of the diodes and put a switch it in. That will change the output from full-wave to half-wave rectified AC. The switch needs to be able to handle the full 200 amp output of the charger, so you'll need to use a heavy duty battery switch. And only turn off the switched diode on the lowest setting; otherwise you're likely to overheat the other diode. And again, it won't prevent overcharging; it will just take twice as long for it to happen.
I would only use this switch on the single 6 volt setting that I have: 6 Volt 40 amp / 100 amp start. Obviously I wouldn't use it for trying to "start", just for timed, slow charging of a 6 volt battery.

Obviously I'm not an electrical engineer, but do understand stuff enough. I have been a mechanic for over 40 years.
 
If you want to reduce that 6 volt charge rate one method would involve modifying the internals (Diodes) like talked about below, but if you're not careful and know what you're doing you could damage the unit. Other methods would of course involve manual disconnection after x amount of time or hydrometer and voltmeter type checks following rest and stabilization NEITHER OF WHICH ARE GOOD ALTERNATIVES to a so called "Smart" automatic 3 or 4 stage charger. You could add some external ballast but it would involve a fairly high power device. I broke down and bit the bullet and purchased Smart automatic 3/4 stage chargers as over the long run they can save manual charging and protect and increase battery life.

John T
 
That sounds like a great charger to use when the tractor won't turn over and you want to use it NOW! If you put it on high
charge for a couple of minutes and then to boost it would probably spin over, as long as the battery isn't shorted out. I
would just buy a small charger-maintainer for long term charging.
 
John,

I thunk about it a while after discussion the solution with Bubba. Bubba said the solution is simple; use diodes in series. Each diode will reduce charging voltage 0.6v. Just add diodes in series until you get the current you desire.

No resistors needed. The solution, use diodes.
geo.
 
> Each diode will reduce charging voltage 0.6v. Just add diodes in series until you get the current you desire.

Hmm. That would be an expensive solution! Assuming there's room inside the charger for all the additional heat sinks you'll need.
 
"Each diode will reduce charging voltage 0.6v. Just add diodes in series until you get the current you desire."

That's not as "far out" as it sounds.

I have replaced selenium rectifiers in the past where the FACTORY replacement rectifier had additional diodes in series to add forward voltage drop, and effectively compensate for the lower forward voltage drop of the silicon rectifiers.

IIRC, I still have a NOS Century rectifier on the shelf that is made that way.

I have have replaced the selenium rectifiers in chargers with 50 Amp bridge rectifiers using half the diodes on each, and have added an extra in series to bring the charging rate back to the original range.
 
Hmm. That would be an expensive solution! Assuming there's room inside the charger for all the additional heat sinks you'll need.

Not really Mark, I have a box full of used diodes, cost $0.

If you reduce the current to just a few amps, no heat sink required.

I recently repaired a 6 amp charger. The diodes were mounted on a circuit board, no heat sink.

As for room, put the diodes in series and to a wire to the ends of series diodes. Clamp charger to one end of wire, and put an alligator clamp on the other end.

Very simple solution. I use diodes in series all the time to reduce the voltage, don't you?
geo
 
Bob,
Every first year electronics student knows silicone diodes use .6v

Other diodes like germanium only use 0.25 to 0.3 V, but you wouldn't use germanium diodes in a power supply.

geo
 
> If you reduce the current to just a few amps, no heat sink required.

In this particular case, we're talking about charger that's capable of sourcing 50 amps or more. You hook up a discharged battery, and your diodes better be up to the job. Note that the transformer output is rectified ac, with a peak voltage of around 9-10 volts. Those diodes need to handle the current spike that will hit them when they become forward-biased. Also, if you add enough diodes to significantly limit the current to a dead battery, the diodes won't forward-bias once the battery is half-charged.

In the case of your six amp charger, each diode has to dissipate less than 1.8 watts at the (optimistically) rated 6 amps. (Since the diode is forward-biased less than half the time, the power is cut in half: 6 x 0.6 x .5 = 1.8.) So they can get by without a heat sink. But six amps would be about the max for non-heat-sinked diodes.

Yes, I've used diodes to reduce voltage. But I also pay attention to power dissipation. Note that a forward-biased diode has significant resistance, which most folks choose to ignore. (A reverse-biased junction in breakdown has almost no resistance, which is why reverse-biased zeners are preferred over forward-biased diodes for voltage references.)
 
Wow, Lots of great info. One thing I've learned in life is that in some situations (this being one of them) I know enough to know that I DON'T know enough, in this case, to figure out correct diodes, heat sinks, etc.
 
"Bob,
Every first year electronics student knows silicone diodes use .6v

Other diodes like germanium only use 0.25 to 0.3 V, but you wouldn't use germanium diodes in a power supply.

geo "

WHERE did I mention "germanium diodes"? I was talking about SELENIUM rectifiers, like THIS:

<img src = "http://www.cehco.com/wp-content/uploads/2016/02/5563C-1-Pic-275x300.jpg">
 

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