How much does battery balance matter?

[BDofMSP]

Hello. Been a long time since I posted - I hope I'm still welcome!

This year I replaced both of my battery banks. Each bank was three G31 FLAs which performed both house and start duties. When I replaced them I added a fourth battery to each bank.

The original configuration of each bank was as attached, with the positive and negative cables connected to Battery 1. I have been told (here) that it's better to balance the bank by having one cable (e.g. starter) on Battery 1 and the other (e.g. ground) on Battery 3, and I was able do to that on the Port bank when I bought the boat. On the starboard, it would have required replacing a cable, and I never bothered.

However now that I just replaced the bank, I'd like to take good care of it. On the Port bank I was able to add the 4th battery outboard of Battery 3, and move the ground to it, so it remains balanced. But on the Starboard I had to put Battery 4 outboard of Battery one, which has the starter and ground connected to it. See attached.

So, while I don't mind replacing the ground cable, it's not high on my list right now vs other projects. My question is, how concerned should I be about this somewhat janky configuration? Can it wait until winter with little impact? Or is it a larger deal than I'm giving it credit for?

Ironically, it was the port bank (which was balanced) that gave out and drove the replacement in the first place. Of course they each have different loads and usage so that's not necessarily very telling, but it didn't seem to have any big negative impact on the starboard bank.

Thanks for any feedback.

BD

 

[tiltrider1]

Move the negative to battery 4 and the positive to battery 3. Now you are balanced.

 

[luna]

I would not run the bank as you have shown, both Bat + and - taken off of battery 1.

tiltrider's suggestion (or do it the opposite way, if it's easier) is a lot better but even it is not perfectly balanced. With a 100 amp load the middle 2 batteries contribute about 13% less current (it's really conductor size dependent) than the outer 2 batteries.

There are ways to make it perfectly balanced but you need to be able to produce good quality battery cables.

 

[alfamike]

do what "Tiltrider1" says to do.

 

[tiltrider1]

I would not run the bank as you have shown, both Bat + and - taken off of battery 1.

tiltrider's suggestion (or do it the opposite way, if it's easier) is a lot better but even it is not perfectly balanced. With a 100 amp load the middle 2 batteries contribute about 13% less current (it's really conductor size dependent) than the outer 2 batteries.

There are ways to make it perfectly balanced but you need to be able to produce good quality battery cables.

Luna is correct. I got you 90% there and the last 10% could be gotten by making all the interconnecting cabLes the exact same length.

 

[tiltrider1]

I would not run the bank as you have shown, both Bat + and - taken off of battery 1.

tiltrider's suggestion (or do it the opposite way, if it's easier) is a lot better but even it is not perfectly balanced. With a 100 amp load the middle 2 batteries contribute about 13% less current (it's really conductor size dependent) than the outer 2 batteries.

There are ways to make it perfectly balanced but you need to be able to produce good quality battery cables.

Luna is correct. I got you 90% there and the last 10% could be gotten by making all the interconnecting cabLes the exact same length.

 

[BDofMSP]

So I forgot to mention a key detail. Battery 4 is about 3 feet away. Either positive or negative will reach 4, but neither will reach 3. Hence the dilemma. If I could reach 3 I would have balanced the last bank. So I can reach 4 or 3.

So I get what's optimal. The question is more about the urgency. Is there any real harm? Or just not optimal?

One other thing. The interconnect cables are 2/0 but I made the cables to Battery 4 3/0 to help compensate for the distance. I just measured and they are about 28" (negative is a bit longer, positive shorter).

Thanks
BD

 

[Bacchus]

If you can reach batty #1 & #2 do it the way depicted.
If not you might consider adding a Pos & Neg buss bar with equal length jumpers to all + & - terminals.
Manybwill argue balancing is not reqd. My feeling is why not attempt to do it optimally? There are usually ways to do it optimally or very close... why wouldn't you try to do it optimal?
The key is to have each batty circuit have the same L cable runs when you combine Pos & Neg cable lengths.

 

[BDofMSP]

If you can reach batty #1 & #2 do it the way depicted.
If not you might consider adding a Pos & Neg buss bar with equal length jumpers to all + & - terminals.View attachment 139573

Yes I think I could reach 2 with the negative, if I move the cushioned clamp that is holding it (I could reach without that but it would be too strained for my comfort).

But buss bar ... Never thought of that. I'll consider that too!

Thanks!
BD

 

[tiltrider1]

If you can’t reach 3 then you can do what Bacchus said.

What you are doing is stepping over a dollar to pick up a dime. Instead of spending the money to replace one of the battery cables you are choosing to wear out two of your batteries at an accelerated rate. You won’t ever notice that you shorten your banks life because no one can tell you how long a bank should last. It’s kind of like never rotating tires on a front wheel drive car. One day you have two bald tires and two almost new tires. The same will be true with the batteries only it’s not visible. Worse, the two bad batteries will then start stressing the two better batteries which will make it look like the whole bank is worn out.

If you want to be cheap, use a buss bar to connect the extra length of cable needed to do it as I suggest in post two.

This is more than just good vs optimal. However, it’s an improvement over how things were originally wired.

As a note I have reached 10 years with my FL16’s and I am just now feeling like my capacity is starting to diminish.

 

[BDofMSP]

I'm happy to replace the cable. A cable is trivial in terms of both cost and complexity. I just have several other projects that I'm focused on right now and my boat is 4 hours away, so I have very little opportunity and have to prioritize. So if I can wait until winter? I'm going to.

That said, I do want to take care of that battery bank. As much as it pains me to have the yard do something as simple as make and replace the cable, I may have to pay them. I leave for a 10 day trip in 2 weeks and we'll be drawing on those batteries a lot. I was happy to consider the alternative wiring option because I have plenty of 3/0 and 2/0 cable and lugs on hand. I do NOT have any 4/0 cable or lugs, which the replacement would be, so if there was another approach, I was interested.

I asked what the impact would be, and it seems that it's not trivial. I'll take some action before our trip and make that right. Thank you all for the input!

BD

Side note - the last bank lasted 6 years. I was hoping for 7 and that is my goal for this new bank. But the whole replacement, including the additional batteries and the new cables, boxes, etc. was only about $2k. If I only get 6 years again, it's about $300 a year vs 200 a year if I got 10 years out of them. And that's including the cost of cables etc. which I won't have to replace next time. The $400 difference over 10 years is almost certainly less than what they yard will charge me to build and replace that cable.

 

[MYTraveler]

Could someone please explain why, if the cables are properly sized (current carrying capacity, not length), any of this matters in applications such as this (ie, not logic circuits, etc.)?

 

[DennisB1]

I have been using the Blue Sea busbars rated for 600amps continuous. i set up battery with all cables of the same length to the busbar (3/8" studs). Each battery should have a ABYC fuse in the area of 300 amps per battery. 2/0 cables would work fine for each battery. You could go to the 4/0 from the terminal block to the engine. Each terminal should not exceed 2 cables per 3/8 stud. With all the battery cable (+ &-) runs about the same length the batteries will be well balanced.
Do you know the current draw for your engine. I have been using on my boat 2/0 cables for Cat 3208 engines without issue (approx 1000 amps cranking)
Also what type of swage are you using to install the wire terminals. A hydraulic swage would work best to keep voltage drop low at the cable terminals.
Blue sea also sells HD battery switches that are rated above 2000 amps intermittent to help keep volt drop low.

 

[Senojev]

Check this site

http://www.smartgauge.co.uk/batt_con.html

 

[BDofMSP]

Thanks for posting that! I was disappointed that my monitor system was delayed in shipping - it would have been installed by now and balance would have been achieved (mostly). Thankfully I have Monday off and it's supposed to deliver then. I should be able to get a good bit of it done.

BD

 

[luna]

Even if all of the interconnection links of a 4 individual battery paralleled bank are the same length, and you connect the loads to the opposite batteries (as is often promoted as the correct way) you do not achieve perfect balancing.

The reason is that in low resistance circuits (like batteries) small differences in battery voltage result in outsized differences in current. The interconnecting links in this parallel configuration do not all carry the same current (some are carrying the current to the other batteries, some are not.) and as a result the voltage drop in the interconnecting links is different. This difference in voltage might seems small but it results in about a 13% reduction in the current to or from the two middle 2 batteries.

In my opinion the best way to achieve perfect balancing is to use the hub and spoke method, with all batteries individually connected to their suitably sized bus bars.

Yes, the "Travel Trailer Tongue" method offered by Bacchus in post #8 provides perfect balancing, but suffers from the fact the you have to stack 2 or 3 lugs on half of the battery posts and the mind boggles with what to do if you have more than 4 batteries to connect into one bank.

Albeit not updated since 2009, (and likely won't be as the author has sold the business) the Smartgauge website referenced above is a valuable resource if you want to understand a few important things about LA batteries.