Robot Pb battery charging question

[bouchier]

Robot Illuminati - I'm seeking advice on a robot battery power/charging conundrum. I'm building the electronics for a robot which has 24V motors but 12V electronics (Mowbot, for those who know him). The natural way to wire it is to put two 12V batteries in series, tap off the lower one & feed 12V from it to the electronics, and wire the upper one to the motor power circuit.

The concern I seek advice on is charging the two 12V lead acid batteries. The lower one will discharge faster, especially during development, because it's running the electronics as well as its share of the motors. If I just hook up a 24V charger to the two batteries I'm concerned charging will leave the batteries unbalanced because the upper one will be fully charged before the lower one.

Is this something I should worry about? If it is an issue, does anyone have any ideas on how to address it?

I imagine the electric R/C guys deal with this, because they sometimes have separate motor & radio batteries. What are the standard techniques for handling this situation?

Thanks for any ideas

Paul Bouchier

[bouchier]

Reply from Kenneth - DaKenengineer-meister

Hi Paul,

A quick question to chanllenge the statement/assumption about the "normal"
way to do it.  Often it can be much more efficent to run both the motors and
the electronics from the +24V.  Use a high efficency step down buck
regulator such as those produced by National Semiconductor.  They make a
line of parts called the simple switchers that will take inputs up into the
36~40V range and can operate down into the battery range you are looking
for.  These parts can source an amp pretty readily.  The next step from
there might be a switching module by TI.  This has the whole switching
'buck' regulator built into a small module, pre-built.

I mention these due to the efficency.  Efficency is directly related to how
long the batteries on your robot will last.  Additionally, it provides two
benefits.  1) the batteries can discharge much further before the regulator
that would have been on the lower battery only will experience a brown out.
Read = longer runing time for the robot.  2) The common type of problems
that people have with embedded processor resets when running motors and
logic from the primary batteries all but goes away when using a very high
frequency buck regulator as the noise in the areas of interest can't pass
through the regulators and associated filters...

-Just something to chew on.  I use this configuration in a good number of
the robots I've built over the years and I get some phenomenal run times out
of them.

-Kenneth

[bouchier]

Reply from Jose Quinones

Hi Paul,

I will be honest here; whether what you attempt to do (tap out of the lower 12V battery that is connected in series with another 12V battery to power 24V motors) is a good idea or not, I have no clue. But there is like a gut feeling which tells me it is not the most efficient way to do this.

I had your problem, precisely! Which is why I came up with the AE-PWR1 board. I think you may still have one of the bare boards. The beauty of this concept, is that all you have is a single 24V power source and the PWR1 generates 2 other voltage outputs. Per example, you could have 3.3V and 5V derived from the 24V and both batteries would be subjected to the same loading. Plus the PWR1 gives you multiple connection points to route your two rails.

Each PWR1 output can support up to 2A loading, which I believe should be more than enough to power up your electronics (unless you are driving a PC on the 12V). I have not tried wiring them in parallel, but I supposed there has to be a way. That would give you 4A of current loading.

Kenneth explained way better than me, but the PWR1 is based on TI's TPS54283 Buck Regulator.

Here is the link to the datasheet.

http://focus.ti.com/lit/ds/symlink/tps54283.pdf

BTW, I can not claim that I do not have affiliations to the aforementioned company, so my apologies if it seems like I am trying to help them, because as it turns out, I am... ;-)

Best regards,

JIQ

[bouchier]

From John Dolocek

Paul,

I can tell you from our work on Sawtooth that two batteries is not a good idea.  In addition to all the things others have mentioned, it boils down to twice as much work when it's time to run.  You have two batteries that you have to monitor and keep charged which adds complexity and can ruin your plan pretty quickly unless you have spares of both batteries.  Sawtooth is going to be converted over to a single li-po battery for this exact reason.  I need to focus on my weak points (programming) and not on maintaining batteries. Just my 0.009 cents worth.

John