Diodes between (-) terminals on parallel batteries. Why?

It's an attempt to prevent a stronger battery from charging a weaker battery. Sometimes used when the batteries are of two different capacities, or wide age difference.
Best to always have both batteries be the same type and age. Then one doesn't need a "patch", they keep each other in balance. Your diagram doesn't show, but depending on where the load comes off, or the charger connects, there could be issues with getting full use, or full charge, out of one or the other. Get two matching batteries, and get rid of the diodes.

Flapper wrote:
It's an attempt to prevent a stronger battery from charging a weaker battery. Sometimes used when the batteries are of two different capacities, or wide age difference.
Best to always have both batteries be the same type and age. Then one doesn't need a "patch", they keep each other in balance. Your diagram doesn't show, but depending on where the load comes off, or the charger connects, there could be issues with getting full use, or full charge, out of one or the other. Get two matching batteries, and get rid of the diodes.

I put two matching batteries in it, so I was hoping I could replace this config with a solid 4 AWG on both the positive & negative terminals. When I dug into it I noticed this circuit and wasn't sure what it did. Thanks for the explanation!

Not only would it prevent a stronger battery from charging a weaker battery
It would prevent the converter from doing so as well.. No valid reason for doing that.

wa8yxm wrote:
Not only would it prevent a stronger battery from charging a weaker battery
It would prevent the converter from doing so as well.. No valid reason for doing that.

The funny thing is it looks almost factory. No mention of it in the manual/schematics, so maybe someone added it.

Never seen or heard of anything like that. How many other wires are on those terminals?

wa8yxm wrote:
Not only would it prevent a stronger battery from charging a weaker battery
It would prevent the converter from doing so as well.. No valid reason for doing that.

I would expect the converter negative lead to be between the two diodes. The voltage drop across the diodes will prevent the converter from fully charging the batteries, though, unless the converter has been adjusted to a higher voltage.

BB_TX wrote:
Never seen or heard of anything like that. How many other wires are on those terminals?

Honestly, it's hard to tell what was factory and what was added later. There are at least two sets of items I added (AC inverter & some accessory lighting) and a line to the Colman Solar charger/regulator that I believe was an aftermarket installation. Other than that the red wire that goes between the positive battery terminals of both batteries (no diodes) and the white wire with the opposing diodes between the two batteries on the negative terminals.

I think "Flapper's" response make the most sense. I think this was an attempt to balance the batteries when one was weaker and/or older and not well matched. I'm going to remove the diode circuit this weekend and replace with large AWG wire between the negative terminals.

Dirtdaddy wrote:
I think "Flapper's" response make the most sense. I think this was an attempt to balance the batteries when one was weaker and/or older and not well matched. I'm going to remove the diode circuit this weekend and replace with large AWG wire between the negative terminals.

That would be my suggestion too.

Also there are a lot of "Myths" about batteries in parallel Some of them are based on batteries in series.

now when you take a pair of GC-2 and put 'em in series to make a big 12 volt they need to be twins. Same everything (Make, Model Age and so on)

But when you put a pair of 12's in parallel.. That' not the case.. The myth is if you put say a Group 24 next to a Group 31 the 24 will "Charge faster" and convince the charger that it's full so it goes to float before the 31 is full.. NOT SO.. or that the 24 will run down faster.. again not so.

What will happen.. For reference I'm going to use 70 AH for the 24 and 130 for the 31 for 200 AH total

The smaller battery will take 35% of the charging current and the larger 65%.. They will hit full in perfect lock step (After the first discharge recharge cycle)

And likewise when discharging they will share 35/65 and hit the cut off point precisely together..> The perfect power commune.

Now in the case of older batteries that have developed a higher internal resistance.. Well then you may have something but they will work it out just not always in perfect lock step.

I am the voice of experience on this.

wa8yxm wrote:

Dirtdaddy wrote:
I think "Flapper's" response make the most sense. I think this was an attempt to balance the batteries when one was weaker and/or older and not well matched. I'm going to remove the diode circuit this weekend and replace with large AWG wire between the negative terminals.

That would be my suggestion too.

Also there are a lot of "Myths" about batteries in parallel Some of them are based on batteries in series.

now when you take a pair of GC-2 and put 'em in series to make a big 12 volt they need to be twins. Same everything (Make, Model Age and so on)

But when you put a pair of 12's in parallel.. That' not the case.. The myth is if you put say a Group 24 next to a Group 31 the 24 will "Charge faster" and convince the charger that it's full so it goes to float before the 31 is full.. NOT SO.. or that the 24 will run down faster.. again not so.

What will happen.. For reference I'm going to use 70 AH for the 24 and 130 for the 31 for 200 AH total

The smaller battery will take 35% of the charging current and the larger 65%.. They will hit full in perfect lock step (After the first discharge recharge cycle)

And likewise when discharging they will share 35/65 and hit the cut off point precisely together..> The perfect power commune.

Now in the case of older batteries that have developed a higher internal resistance.. Well then you may have something but they will work it out just not always in perfect lock step.

I am the voice of experience on this.

This is interesting stuff! It makes me think I could put a third battery in my trailer without too much fuss ( it would be a newer version of the same ones I bought at Costco). thank you for the help