I’m doing a little project where i’m replacing an old 6v lead acid battery (a Yuasa Np4-6W to be exact) which I don’t believe is holding a charge any more, with a newer Lithium-ion battery pack which gives an increased Amp hour as well as much quicker charging rate. However, there are a couple of problems.
The first I think I can overcome without much trouble, and that is the battery pack is 12V output, while the lead acid rechargeable is 6v. I was planning on trying to do a basic resister drop with two 100ohm resistor’s (one on the positive, and one on the negative) to drop the 6vs and hopefully not produce too much heat in the process. waste case, just half the values and increase resistor count. Probably not the best method but should be fine (famous last words?)
Anyway… the main issue I have is the connections. the Cell does have charge protection built in, so that isn’t a problem, but is a cell that has two connections. a barrel jack to plug for the load, and a socket for the charging. The original battery only has the one jack, for both charging and load. without doing some slightly fancy (only slightly) edits to the housing, I was wondering about combining the two into the one connection. but I’m not quite up on the work needed for this. As the project I’m using, has a socket for charging, I would guess (without taking it apart, which will probably be a later stage for the project), I would have some circuit for swapping over from charge to discharge state… but that doesn’t really help me on this side… I don’t think I could just connect the two sockets on the battery together… Though maybe with a couple of diodes and reversing the polarity to deal with it? I’m very much basic level of circuits and don’t know many people these does who could help out…
First, using resistors to reduce voltage won’t work unless your load is always constant. If it varies at all, the power supply voltage will change. An added disadvantage is that all that voltage you’re losing in your resistors gets wasted as heat so if you start with a 12 volt battery and get 6 volts out the resistors, you’ve wasted half your batteries capacity heating resistors and not powering your load.
Second, your device probably has some sort of charger built in that charges your old lead acid battery when it has AC power and you’d need to disable that somehow to make it work with a lithium ion battery. Trying to charge a lithium battery with the wrong charger can cause the battery to fail in a very bad, very fiery way.
Lastly, you’d need a charger connected to the battery somehow to charge it since your device can’t. So now you need additional circuitry besides modifying the circuitry inside your device. It might be easier and less expensive to just stick with lead acid since it’s cheap, reliable and easy to recycle.
If anyone else had ideas or can think of something I missed, please reply. I’m not an expert and there could be information I’m not aware of.
TS-Chris:
This might be more trouble than it’s worth.
I agree.
It would be helpful to know what sort of device is being discussed or at least get an idea of how much power is needed. Some sort of regulator would certainly be better than the proposed fixed resistors, especially given the low capacity of the existing battery. The charging issue is likely to be a problem.
Lead acid batteries are pretty good when weight isn’t a major concern. Also, one should be skeptical of rated capacities on a lot of lithium ion battery packs. They’re often, at best, optimistic.
It’s an old Classic Omnibot I’m not looking to do extreme mods like some people do, way out of my range, but I’m just toying around and having fun.
I know you what you mean about capacities varying, and these aren’t pricey cells really, but I tried using my capacitor measurement circuit and it’s pretty within range.
I had hopped the 6 volts, while dropping it is a real pain, wouldn’t product too much heat if I spread the drop over a few resistors but, yeah, might not be great… I was more thinking of just unconnecting the inbuilt charging (which I would need to strip this a bit to look at the circuit work inside)\ which I was planning on later anyway). I’m well aware of the problems of lithium cells, and normally use fair enough checking and control when I can, or at least what I think is fair enough. I wouldn’t be using much but a decent good charger for it.
While I wanted to see if I could just keep the cell as a simple plug in, if I needed to, I did think about just extending the charger connection cable, so it runs to the old socket so the charger socket was connected directly to the charge of the battery (unpluggable still via an added socket), and then the one going in. As I said, the Battery pack I got has a built in charge protection circuit and like I said, currently has the two connections, a barrel jack for the load, and a socket for the charge.
never really thought of the lead acid cells are being totally fantastic mostly due to not having a very long life span (don’t they normally stop a charge after about 11 years or so?) and they seam to have a very long charge time.
As for more trouble then worth… mm… debatable… When someone says “Why make this basic circuit when you can buy it and save time and money”, making the circuit is often way more fun.
just remembered I forgot to say, while the Battery is 6volt, and it takes a 6v (centre negative which was a minor pain) charger adapter, the Omnibot itself says it runs on 5.5volts, so a simple enough tolerance there.
It seems like a perfectly doable project as long as you recognize some dos and don’ts.
There are some motors but I can’t imagine there’s more than a few amps drawn. The lithium pack will probably work as long as you regulate the output voltage down and only charge externally. In other words, do not use the robot’s AC adapter or the robot’s charging connector when the lithium battery is installed and only recharge the battery using what comes with the battery pack.
Here’s a switch mode regulator that can supply up to 5.5 amps:
I was thinking of re-wiring the AC socket to go to the new pack and yes, I don’t have the original charger, and I know enough to get hold of and deal with a good replacement (boy, the manual has a fair bit of information on that stuff… more then I would have expected).
Looking at them regulators, well… if they say the 4ah battery lasts about 4 hours of continuous use at 5.5volts, and the fuse is 5amp so… … fuse doesn’t matter, it’s probably about 1amp current if I measured that right… I don’t have any 6volt regulators… though I have some 5volts, I think 7805s… So… use the IC, a couple of diodes on the ground pin… a couple of capactories and I should be able to built the step down regulator circuit… I have no clue why the diodes on the ground pin increase the IC from 5 to 6 volts but the circuit plans I got seam to say it does… basically… 12volt in connected to a 0.1uf cap, which is connected to the ground rail, and to the in pin on the IC, the ground connected to 2 diodes pointing towards the ground rail its connecting to, and the out on the IC connected to another Cap, and to the 6v rail… should be good for about an amp… I guess, since the thing has a fuse, I could built the circuit and test it out… worse case, I blow the fuse
okay, for now, i’ll just replace the lead acid battery… I tried the circuit, I screwed something up (not 100% sure what ^_^:) and the 12 volts went in, 0.7 went out… and of course, this really heated the IC didn’t even try to plug it in to the bot…
As long as you use an appropriate charger and don’t try to charge the battery in the bot, a 2 cell lipo battery (7.4 volts) MIGHT work OK on it’s own without any voltage regulation. There is the possibility the extra voltage (8.4 volts when fully charged) could cause damage to the bot though.
If you wanted to play it safe, I’d put a [switching regulator between whatever battery you end up using and bot just to keep the output voltage at a steady 6 volts. The linked regulator would be good for 5.5 amps and as long as the bot doesn’t need more than that, you’re probably OK. You will need to remove the battery from the bot and use a charger designed for that specific battery to recharge though.](Pololu - 6V, 5.5A Step-Down Voltage Regulator D36V50F6)
I’m not looking to do extreme mods like some people do, way out of my range, but I’m just toying around and having fun.