View Full Version : Battery / AC all in one

03-13-2008, 09:13 AM
I have a couple of battery packs in a robot. I'd like to be able to charge the battery packs, and have the robot powered on (via AC power) at the same time. In addition I'd like to be able to remove AC power and it switch to battery power immediately (without having to power down the robot (which is controlled by an on-board pico-itx motherboard) ).

I'm using 2 12V battery packs in parallel and I have a DC-DC converter (12V input) for the pico-itx. For reference, I also have a voltage regulator turning some of that 12V into 7.5v for a SSC-32 servo controller powering 23 servos (HS-475 and HS-322) coming right off of the battery packs before it hits the DC-DC converter.

I would like to think it's as easy as putting in an AC power plug in parallel with the batteries, but I seriously doubt that. I know there's a bit more to charging a NiMH battery than pumping juice into it.

I would also like to integrate some way of monitoring the battery charge. I've got a Phidgets 8/8/8 that will be in the mix somewhere, so I'll have a couple of analog inputs. In short I want it to perform like a laptop.

Any ideas?


03-26-2008, 10:58 PM
Hey James,

I'm pretty sure you will need to remove any load from the batteries before you start charging them, so you need a system to switch the main power from the battery to another power source. Check the following link out:


The website is using SLA batteries but the ideas are the same. For my own PC robot I'm going to be using the same power management module and charging/docking station design. Read through the readmes for the PMM and the docking station where you'll find a bunch of great implementable ideas.

03-30-2008, 06:00 PM
Sweet, thanks! That gave me exactly what I needed to start.

I ended up using a DPDT switch to cut off the battery from the robot circuit and redirect it to the battery charger plug. I put a barrel plug for wall outlet power in to allow the robot to run on that while the robot is charging. For an instant after I plug in the robot and before I flip the switch the robot is running off of a battery and wall plug at the same time (in parallel). Do you think that would hurt the battery?

It's not a docking station solution quite yet, but it's getting there.

03-31-2008, 01:25 PM
Hmmm, I'm fairly certain it won't be much of a problem. I might see an issue if the battery is discharged to a point where the voltage difference between the battery and the external supply is high enough to cause an unwanted opposite current flow across the battery. I guess you could use diodes (rated for your use) to prevent this from ever happening though.

In any case, make sure you are using a fuse in series with the battery to prevent anything bad from happening :)

04-04-2008, 10:57 AM
I tried the diode route, but they drop the voltage 1.1V, which doesn't work for me. I've been running the robot just switching back and forth with a DPDT switch with no issues. I did have a mishap because I'm not using a fuse (everything still works though :) ).


04-15-2008, 12:17 AM
The big potential problem I see is if you do it whilst motors are running. Then you're risking damage from inrush current. I'd switch my DC-DC to something that protects against inrush current.

I designed a mobile DVR that kept dying switching over to AC with the battery in. Turned out the HD was reponsible for tremendous inrush current - surprisingly so. I located a highly protected DC-DC regulator and my dead DVRs went away immediately.

05-13-2008, 07:52 AM
I know this is an older post, but someone may stumble upon this in the future.

Whenever I design any system I always have a power filter/step-down circuit between my system and the power source. This makes it easy to change power sources down the road (go to a bigger battery or higher rating AC power supply). Also if you integrate one or more capacitors you can set them up to be a backup power supply that will keep you running for one or two seconds. While one or two seconds may not seem like a long time, it is plenty for switches/relays to flip and keep your system up while your power source changes from battery to AC. Using this kind of a circuit also gives you more flexiblity in changing power sources later, perhaps a small generator or even solar cells to reduce/eliminate battery drain.

This is just my preference for designing the circuits and it has saved me loads of time (and money) when power sources I initially used are no longer available or a cheaper but different spec alternative becomes available.

05-13-2008, 11:15 AM
Thanks for the info SkipFire! No worries about replying to older threads here in the TRC; it's all about helping out the community:D

05-18-2008, 08:43 PM
I'd love to see an example of the circuit you're talking about. I have an issue with switching from DC-DC to AC-DC because of either an in-rush issue or something in the AC-DC transformer I'm using.