I'm currently designing the electronics for a robot I'm building that has a
36V main drive motor. I was planning on using 3 12V batteries and was
wondering if there is anyway of getting 36V from the batteries in serial AND
12V from them in parallel at the same time?
I'd rather stick to just these 3 12V batteries instead of havng two seperate
battery packs but I expect the answer's going to be no and so I'm going to
Actually, having the control electronics powered by a separate supply
is a blessing. It helps to keep motor noise from infecting your
controls, makes troubleshooting easier, lets you optimize the relative
sizes of the batteries and keeps the computer running even when the
motor power is gone.
In addition to Joe's comments about interference, if the draw from the
tapped battery is more than a few percent of the total amperage it means
that battery will drain faster. It could drastically reduce the
available play time.
Assuming the 12 volts is for a low amperage application you can use a
switching regulator to take the 36 volts to 12 ... a linear regulator
will just waste a lot of heat. Switching regulator chips are pretty
cheap these days.
If the 12 volt tap requires some hefty amperage, then I'd also recommend
a separate battery. Might be easier than building a high-current
switching regulator. Get a double-pole switch to turn them on and off
together. You will be adding more weight with the extra battery, though,
so take this into consideration.
also if you only charge them in series the tapped battery will not
recieve any more charge despite the increased use. this means that you
only ever fully charge that one battery by overcharging the others.
this is almost certainly what you want to do.
or, if you're feeling clever and using lead acid batteries, build a
dc-dc converter with a variable output that tracks input voltage
divided by three. you can intermittently source as many amps as you
want from the 12v circuit and the dc-dc converter will keep the three
that solution would be ideal for powering a 12v starter motor from a
36v bank, for example, but for constant load like electronics you'd be
better off with a switcher.
It seems that both of these guys failed to answer the original question
(unless I missed a post).
No, there's no easy way to connect 3 batteries in parallel and in series at
the same time - as you suspected.
But what you can do, (and what both of them were talking about) is simply
connect them in series to get your 36 volts, and then use one battery out
of the 3 for your 12V needs. So if you have a negative common ground
between the 12V and 36V (which would be typical), you just use the + side
of the battery connected to the ground as your +12V source. Maybe this was
already obvious to you? But if not, I thought it might need to be
The problem, as pointed out, is that the one battery doing double duty
would be drained faster which leads to issues - but if your 12V power needs
are very low compared to the 36V needs, then it can work. You could also
just use a slightly larger battery for the one doing double duty. But that
might create more issues with the stronger battery reverse charging and
damaging the smaller batteries when they run down before the larger battery
does. I don't know how much of a risk this is with batteries.
The best options if you need any real 12V power is either to use a separate
battery for the 12V, or use a DC-DC converter to convert the 36V to 12V.
Thanks for the replies. The robot is actually a AUV and so I'm trying to
keep voltage regulation down to a minimum as it's going to get nice and
toasty in there anyway.
The 12V is to drive auxillary motors and not actual electronics (that will
be on a seperate 12V battery system using car pc electronics for DC-DC
conversion(unless that would cause a problem?)) and with around 10 'medium
sized' motors running off this one battery on and off I just didn't want to
add yet another battery. 5 car batteries is going make this one heavy 'bot
What it be best to have a 'smart wiring' solution where the battery with the
highest voltage is connected to the motors. (Motors won't be running 24/7
just the odd little burst to open doors etc)
If you are connecting multiple motors maybe you can have different taps
for different motors, to balance out the loads a little better.
Not sure what Curt meant about it being "difficult" as it's not at all
difficult to wire up batteries in series and in parallel. In the old
tube days it was sometimes done on cheaper equipment to obtain various
screen and heater voltages. However, whether it's a workable idea or not
depends on the load balancing of the batteries, and the current draw of
I definitely would not do ANY of this if these aren't good ol' lead-acid
batteries. All bets are off if they are lithium ion. Be sure to provide
adequate fusing for each battery on the tap, and for the series as a
Do look into other alternatives that will probably give you better
service. Check out some of the ready-made DC-DC converters. The added
expense might be worth not having to mess around with separate charging
of the batteries, etc. There are also taps/balancers for solar panel
systems you should investigate. They are designed to provide 12v from
multiple 12 volt cells connected in series, and balance the load across
Yup, Gordon they're going to be lead acid....
At the moment I'm not concerned with the charging.....if push comes to shove
I can just disconnect them from the system and charge each one individually.
You would only tap off one of the batteries for the 12 volts, though a
load balancer (available commercially for things like RVs and solar
panel systems, or you can make one; I've seen plans here and there)
would allow you to parallel the three, while still having them in
Yeah, you mentioned that idea in your other post. How exactly does this
"load balancer" work? What is its basic theory of operation?
Hey, you can do this with diodes can't you? Configured much the same as
you would configure a full wave rectifier but using 6 diodes total to
connect the the 3 batteries in parallel to a single 12V load (of course you
loose some voltage due to the drop across the diodes, and you need diodes
large enough to carry the load, but it looks to me like it would work and
it would just naturally keep the the batteries balanced (the one with the
largest charge and voltage would naturally deliver most the current to the
12V load). Is this done? And if not, why not? Is this the "load
balancer" you are talking about?
I thought about this when the question was first asked but my quick look at
it seemed like it wouldn't work - I thought it would just create a dead
short. But looking at it now it looks like it would work.
Do they make diodes that can handle these sorts of current loads (10 to 50
amps) at a reasonable price? (and if the 12V current load is low (like 1
amp), you just use low cost diodes).
And if you wanted to charge the batteries with a single 12 volt charger,
you could connect another 6 diodes in the reverse direction to hook the 12
V charger to the batteries. And again, it would naturally balance the
charge load across all three batteries giving the most charger to the
battery with the lowest voltage.
So what are these load balancers if they are not just diodes?
To answer some of my own questions, I found stuff under the name "battery
Here's a white paper titled "Switched Capacitor System for the Automatic
Series Battery Equalization" I found interesting.
The same company sells battery equalizers on that web site as well that
work using this technology. (but only for 2 or 4 batteries and not for 3).
And here's a product the original poster might be interested in which
converts 36V to 12V (both directions) (couldn't find any technical
information on how it actually works):
It could be used to drive the 12V aux motors from the 36V string for
Or, he could use battery equalizers to equalize the charge on the batteries
and then drive the aux motors from just one battery in the string - the
equalizers would then redistribute the charge as needed.
Here's one such equalizer (he would need to use two of them for the 3
It has a 2 amp max equalization current so the average aux motor load over
time would have to be low enough that the 2 amp equalizer would be able to
keep up. They also equalize the batteries as they charge.
I also looked at diodes and found them in the 8 amp range with a forward
voltage drop of about .6 volts which wouldn't be bad for about $7 each
(without looking for deals):
Answering another of my own questions, I see this would not work. If you
connected the diodes as I was thinking, you would just end up with the full
36 volts less the forward voltage drop across the 12V load. Current would
flow in two of the 6 diodes and the other four would just be reverse
biased. The diodes would be pointless.
And if you tried to do the inverse, to charge the battery string with a 12V
charger through the diode array, the 12V charge would just get shorted out
by the diodes - not good.
So it requires something more complex with active elements to make the
I'm not sure how they work, though at $150-200 or so for a store-bought,
I imagine there is some hefty electronics in it. I doubt it's just
Going back to the OP needs, I had assumed a tap from a single battery.
As a kid I made a couple of tube-based kits that used 22.5v batteries
this way. The power was pulled from only one battery at a time; they
weren't paralleled together. I see now that the OP used the terms serial
and parallel, but I didn't consider paralleling the three batteries to
obtain 12v at a higher current, given that he didn't talk about needing
the current, just the lower voltage.
So the answer would be it's "easy" to tap off one of the 12v batteries
to get a source of 12 volts, but as noted before, it causes a load
imbalance where that batttery is discharged faster than the other two.
This diminishes battery service, and if the battery with the higher load
is over-discharged, which is a possibility, it's ruined.
When you say hard limit do you mean using hardware AND software or just an
I'm curious to see what other people in the group think about
this......Anyone strongly disagree with this providing there are various
'failsafes' to prevent going over the 33% mark?
Depending on the motor it may not even run (or at least not start to
run) at anything under 50%, so you need to test with the motors you use.
If these are for intermittant use you might be able to crank up the duty
cycle a bit, at least to start the motor.
The best failsafe is good programming of an MCU delivering the PWM. You
might also put a fuse or fusible link inline with the motor that trips
at a dangerous current level for X period of time or longer. If the
motor does get the full 36 volts, its current consumption will also
increase. This is something you can test for, and build in adequate
Watch out for are the peak currents during start-up and while under
heavy load, and the correspondingly large switching transients. Also,
there is the risk of magnetically saturating the motor core, so the
current-limiting circuitry is critical. This is not a project for a
I have no experience with this trick, but most the voltage limits on motors
have to do with how much they will heat up if you try to push too much
power through them. So limiting the power by limiting the duty cycle is
likely to work just fine.
The only real issue is if the insulation or contacts in the motor might not
hold up to 36 volts instead of 12. My bet is that this trick will work for
most motors and at worse, you will just limit your motor life a little bit.
You will of course need H-bridges designed for the higher voltage.
The other idea that seems reasonable to me is to put your 12V aux motors on
your 12V electronics battery. You risk interference issues, but the added
filtering to solve that might be better than adding a 5th battery.
And the other idea, is to buy all 36V volt motors for your aux needs.
BTW, if you are using 12V car parts, they tend to have their cases grounded
to one side of the motor - so if you are going to to play games by trying
to drive these motors off of different taps from your 36V battery string,
you have to be real careful with insulation not to create short circuits
through the grounds or else you will see some real sparks and melted wires.
It's very dangerous to have a motor case with a different ground than the
chassis or other metal parts - one dropped screw driver could melt half
your wiring and blow out a lot of electronics.
That could work. But seems overly complex.
Also, as I mentioned in the other post, if you are using motors designed
for use in cars that turn only one way, they can have their case connected
to one side of the motor, which would prevent you from having your 36V
string grounded as well if you want to switch the 12V aux ground between
different batteries in the 36V string. So that's something else to watch
The cleanest design is probably to drive everything from the 36V string
using DC-DC converters.
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