common ground for 24 and12?

I have separate 24 and 12 volt systems, can the negatives be tied together?
Thanks!

rstrange

Re: common ground for 24 and12?

Why do you want to? With no other information, I'm hesitant to offer any advice.

Re: common ground for 24 and12?

Silvertip said:

Why do you want to? With no other information, I'm hesitant to offer any advice.

Click to expand...

OK, I was hestitant to get into all of it, but here it is: I have a 24 volt trolling motor system (2 size 27s) and a starting battery (1 size 24). I have a Truecharge onboard 12v charger for the trolling motors (I used to have a guest onboard charger for starter, but burned it up using a quick start machine). I can charge my 24v system with a 12v charger because I rigged up a switching block with 4 auto starter relays that switch the trollers from series to parallel with a flip of a toggle (it works!). OK, since I burned up my dedicated starter I notice that there are three pos lugs on the Truecharge and one neg. Currently, I have the neg and two pos connected to the 12v side of that switching block ( I guess I could have used just one). Anyway, I was hoping to avoid the purchase of a dedicated starter charger by tying both systems to a common ground and running the third output of the TC to the starter.

Kinda complicated,
rstrange

Re: common ground for 24 and12?

Let me clarify my statement above. When I say, "OK, since I burned up my dedicated starter I notice..." I mean, "OK, since I burned up my dedicated starter *charger* I notice..." . Also, all the batteries are AGMs. BTW, the quick start machine also melted the plastic case on the old AGM starting battery, no harm there as the battery was about dead, but too bad about the $100 Guest charger!

Re: common ground for 24 and12?

I suggest you draw a diagram of what you have and post it as a picture.

Re: common ground for 24 and12?

[colour=blue]The short answer is no. The long answer depends on the above mentioned diagram.

Re: common ground for 24 and12?

No comment unless I see an accurate diagram.

Re: common ground for 24 and12?

OK, guys here is the pic:


I lay awake last night thinking about it and I see no reason it won't work, I'm not really going to tie the grounds together, just connect them both to the Truecharge. When the Truecharge is on the TM battries are 12v and the TC is designed to charge three 12v batteries at a time. When the TM batteries are in series, the TC is off.

rstrange

Re: common ground for 24 and12?

Quick clarification, the solid lines above have been in place for years and the system has worked well, my question is about adding the starting battery to the mix.

Re: common ground for 24 and12?

Again, until I see the wiring on the trolling motor end of this setup I will not provide any advice. But -- Look at your wiring carefully and you will see that you indeed are connecting the grounds together and all I see are "potential" issues. First, the charger does not have isolated outputs since I see a common ground. Using a common ground on a switched 12/24 system can cause a meltdown if for some reason the 24V system does not automatically switch fully out of 24V. The problem connection is the positive to negative series connection between batteries -- regardless where that connection occurs. Modern chargers have isolated outputs which means you have one, two, or three totally separate chargers in one package. Therefore the 24V series connection does not need to be broken when charging. One set of outputs simply feeds each battery. There is no common ground. When trolling motors are fed by two sets of 12V wires, there is normally a switch panel at the front of the boat, on the motor itself or both, or there is a jumper on the back of the receptacle. Since I have no Idea which system you have, I again can only caution you about what "could" happen.

Re: common ground for 24 and12?

Thanks for your advice Silvertip, the 12/24v switch is on the trolling motor itself, but the switch died and I just tied the wires permanently in 24v position (never much used 12v, anyway). Your point is well taken about the common ground shortening out the batteries in a 24v configuration. I looked back under the console at my "creation" and I'm not sure I represented my device correctly in the drawing, the grounds are indeed strapped together by the common charger ground, but my switch must totally isolate the charger in 24v mode or I would have shorted out the batteries the first time I swtiched to 24v. Remember this was 7-8 years ago and I don't remember my thinking at the time and while the wiring is neat I can't remember whether the four relays are single pole or double pole or what. I now think that the relays switch the batteries between facing the charger in 12v mode and facing the TM in 24v mode so TM and charger remain isolated, they have to be, I think. Fact is, it has worked flawlessly for years and with my luck, it must be pretty fail safe!

Anyway, I can't expect you guys to understand my system when I don't!!

Thanks again to Sivertip and others for reviewing this problem, it is a head scratcher!

rstrange

BTW, the Truecharge is late model, top of the line 4-stage charger with auto profiles for AGM, gel and flooded batteries, that is why I use it on the big trollers, it is a much more sophisticated charger than say, a 12/24v Guest with separate grounds. It is just that the TC was designed for cruisers with big 12v systems, not anglers with trolling motors.

Re: common ground for 24 and12?

OK, if anyone is still reading this thread and has not given me up as a lunatic, note that I have altered the drawing, above, I posted this morning. The relays switch the trolling motor batteries between the trolling motor and the charger, They are connected to one or the other depending on position of the switch. The trolling motor and charger are always isolated. It seems to me almost for sure that I can connect my starter to the charger too. In fact, I could even charge my starter with the trolling motor batteries swtiched to the TM.

Please wave me off, if I have missed something, and thanks for you patience!

rstrange

Re: common ground for 24 and12?

Looks fine to me as long as you can guarantee that all 4 relays switch at exactly the same time when in 24V troling motor mode.

You can also ditch the two relays controlling the ground/positive of the battery that is the bottom of the 24V chain.

You of course can avoid all of this relay headache by buying a battery charger that has independent grounds for every battery. Then it wouldn't matter if two batteries are in series for 24V mode. I checked the manual on yours and, unfortunately, it uses a common ground.

Re: common ground for 24 and12?

Don't take me wrong on this -- I'm not implying it won't work as you've shown. I'm only pointing out that electrics can fail. Relays can and do stick closed and as Bruce pointed out, may or may not energize together which could cause an issue. Since you feel confident about the system, give it a whirl. I just hope you have everything fused/breakered though.

Re: common ground for 24 and12?

Thanks guys, your points are well taken. I have chosen the "hard way" to charge my trolling motor batteries, it was kind of a "one thing led to another" that had me putting the Truecharge in a application it really wasn't designed for. I also, recognize that things that can fail often will, but I have had good luck with the system so far and will stay with it. Relays usually stick closed (toward the charger in this case) and if any one relay were to stick shut I think the only problem would be a trolling motor that wouldn't run. I also see now that the bottom ground leg switch is unnecessary, but I was just being cautious.

Thanks again, these forums are the best thing about the internet.

rstrange