I’m certainly no electronics expert, so I wanted to see if anyone had any input on this.
Lets say I have a 6v battery hooked up to my SSC/Servos. Would I be able to have a wall jack connector/power supply ( lynxmotion.com/Product.aspx? … egoryID=44 ) hooked up to it in parallel and be able to switch it on first, then disconnect the battery so that power wouldn’t be disrupted? Or would this damage the electronics and/or batteries?
Thanks!
Hey,
I’m guessing you can make some sort of relay circuit. You can set battery to priority. And if the relay doesn’t get current from battery it switches over to the wall jack. How to do this, no idea as I haven’t used relays. I’m sure you can find something on google. Actually, i’m going to be making one of these my self when I start work on my project.
-robodude666
So simply switching on the power supply, then switching off the battery using toggle switches wouldn’t work?
Basically my question is: would having both a wall power supply and battery turned on in parallel attached to a SSC/Servos, damage the SSC/Servos or batteries? They wouldn’t be on simultaneously for more then 5 seconds at a time in this case.
Well, if you have two power sources in parallel, you just increase the amperage.
Say you have two batteries, 12v @ 2A. If you series them you get 24v @ 2A. But if you parallel them, you get 12v @ 4A. Higher Amp output shouldn’t hurt the SSC/Servos. Only voltage really would.
If you have a relay switch of some sort, the wall wart can be on 24/7 but it won’t power anything because the relay switch would detect current from battery. As soon as the battery dies or gets low (You can probably adjust threshold via circuit) the battery will be switched off and turn on the wall wart to power the SSC. Its like an automatic switch. You can make this with a regular 3 position flip switch I think. But a relay would make it switch automatically so that power is never lost.
Cool, thats what I thought.
I actually want to set something up for the onboard pico-ITX pc im setting up… so that when it is running low on power and I want to keep working on something, I can just plug it in to a wall source and unplug the battery for charging.
I guess I just need to figure out what the maximum amperage input is for whatever electronics I’m going to be powering, and for the picoPSU I believe its high enough to not worry about going over.
Thanks for your explanation!
There are several picoPSUs. Some are 6-12v, some are 12-32v. But they all are 2-10A. If you go over, its BAD!! Make sure you get a “W” (Wide input) picoPSU.
I’m probably just going with the picoPSU-80 model, its low cost as it only has a 12v voltage range, but thats all I will need for it.
And I assume that the batteries wouldnt be effected by a power supply placed in parallel with them? It wont try to ‘charge’ them? I seem to recall that charging isn’t done in parallel, but rather in series, but its been too long and I dont quite remember my electronics basics so well these days.
I don’t know how charging works either, as I never bothered to check because I never needed to make a charging circuit for a battery.
It is better to get a wide 12-32 or something because then you can use 14.4/14.7v which are more common in lithium based batteries. If your using NiMH or whatever then it won’t matter.
Hmmm, excellent point. I might want to upgrade batteries at some point.
I dont know though, I have heard scary things about Lion batteries, and dont really understand how they charge differently, so haven’t really considered them too much.
Li-ion computer batteries have protection circuits in them so they are difficult to over discharge or over charge, unlike raw LiPo cells used in the r/c airplane crowd. Note however that a great many of them are limited to around 3A-4A max output unless you specifically buy high discharge rate packs which are in the 6-8A range. Also an important point to note is most of them have short circuit protection fuses in them and if you manage to pop it you can send the battery to the recycle center. I have a Molicell manufactured DR-202 clone (11.1V 6600mAHr flat pack) sitting here as a testament to that very fact… makes a nice paperweight. 
Tyberius, this should be fine! This is what you would be doing if you were charging the battery. If you left it connected, eventually your battery voltage would equal that of your power supply. But then batteries should be sharged to higher than their nominal voltage, so I would still charge your batteries the way you usually do.
I just want to also clarify that you don’t increase the current into a device by adding a power source in parallel. ‘amperage’ is dictated by the device. As long as you have the correct voltage, and your power supply can supply at least the current the device will draw, you will have no problems. You could hook up a million batteries in parallel and you would not have a problem… just a very long lasting battery. 
Next, robodude666, I think your idea of using a relay to change power supplies may not work if you are running a microcontroller that you don’t want to reset. Relays are very slow. By the time the connector got from one pole to the other, your MCU would have almost defenitely reset. You may have to look at other options. If it is only for the power of servos and motors though, you should be right.
Hope this has clarified some stuff.
Hey Steve,
You are totally right. I haven’t used relays really and I forgot that they are rather slow. Would adding a super-cap in between the relay and micro help? The super powerful cap could power the micro while the relay switches.
Right. Well, I was hoping he knows that the load uses as much current as it needs. My example simply shows that the load can take upto 4A of power when the battery is in parallel. And if you have two 2000mAh batteries in parallel, it will last 2x as long because you have a higher mAh rating for the entire battery.
-robodude666
Thanks guys! That clarifies exactly what I was wondering about. I wasn’t sure if there was such thing as too much amperage however. Thanks again
The problem with using a super-cap is that the charge would be used up very fast by the servos and other stuff you might be running.
The question was asked a little while ago, when looking at a kind of DIY UPS from just one source, if the voltage regulator whould allow reverse current.
If it blocked reverse current, then you could put the cap after the regulator and the power wouldn’t be ‘stolen’ by the other components. The problem then is that it will only be charged to 5v, so there is less room for voltage drop before the MCU resets. But I say that without knowing exactly how much charge the cap can hold, and whether the regulator can even do that (I don’t think anyone could answer). I may check this at some stage as I have heaps of regulators floating around here. It’s a complicated little problem.
I think the best thing to do would be to power your MCU from a different source. However if you do try a super-cap, let us know how it goes, it would be interesting to see if it does the trick.
