Inter-battery connections of Series/Parallel battery banks

DIY
, , ,
1

Hi folks. I’m implemeting an Orion DC-DC converter (24/12, 70A) and BMS on a series-parallel lead acid battery system in an RV. While researching how to get this install done, I have read Victron’s Wiring Unlimited publication with great interest and have implemented some changes to the RV battery system and wiring as a result. One question that remains is whether to interconnect individual batteries within each series banks to its mate in the other series bank.

First some background… The house battery system in this RV is made up of quantity 8 6Vdc lead acid batteries, wired such that there are two parallel strings of 4 batteries in series. The net result is a series-parallel 24Vdc system. The RV manufacturer chose equipment to charge at 24Vdc and use midpoint take-offs to feed the 12Vdc load. Not ideal from a system capacity perspective or battery life-span perspective, so the system has now been changed such that:

-the majority of 12Vdc load is fed from an Orion 24/12 converter. There is a small amount of load that remains connected to the mid-point of the 24Vdc, but its use is extremely intermittent and does not draw any parasitic load when not in use (that is, electro-mechanical contactors are in circuit that positively disconnect loads when not in use). And it would be a great big deal of inconvenience to re-wire those three loads in any other way.

-the two banks of series connected batteries are now wired ‘diagonally’ to form their parallelled connection to the remainder of the connected equipment like MPPT charger, 120Vac-24Vdc inverter/converter/charger, 24Vdc engine driven alternator and connected loads. Note that the batteries are not all located in one area within the RV. There are 6 batteries at the rear, and four of these form one series connected bank and the other two form half (that is, 12Vdc) of the other series connected bank. The remaining two batteries in this second series connected bank are located at the front of the RV. The runs from front to rear of the RV use 4/0 fine strand welding cable, and the series connections between adjacent batteries uses 1/0 fine strand.

The RV manufacturer chose to install additional interconnect wiring between each battery in its series bank with its respective battery in the other bank. Batteries A, B, C and D form one series connected bank and batteries E, F, G and H form the other series connected bank. Batteries C and D are the two that are located at the front of the RV. The existing interconnect wiring runs between B+ & F+, C+ & G+ and D+ & H+.

So my question is whether to completely remove the interconnects, OR leave all interconnects OR leave only the mid point interconnect (C+ & G+)?

I don’t recall seeing these scenarios detailed in the Wiring Unlimited tech document. And apologies if I missed that, and please do point out where I can learn more about the benefits and consequences of removing or leaving some or all of the interconnect wiring.

I welcome all comments and questions, thank you for your consideration and look forward to learning more from your input.

Cheers,

/paul

2

See page 19 and 20 of wiring unlimited, Section 3.5.

3

Thanks for your input @pwfarnell.

I did see that info in section 3.5, however it did not sink in with me. I guess there’s a couple reasons for that - I’ve got 2 parallel strings of 4 batteries (whereas section 3.5 shows 3 parallel strings of 2 batteries); I’ve got 2 of the 8 batteries located elsewhere in the RV (resulting in special consideration being given to inter-battery cabling and ruling out the possibility of bussing).
Here’s a schematic of the presently installed equipment. Keep in mind that the 12Vdc load connected to C+ is very intermittent/infrequently used. The house’s main 12Vdc load is connected to an Orion DC-DC that is fed by the 24Vdc shown at the bottom of the page.

Batt_Schem
Batt_Schem1920×1815 591 KB

The goal of this exercise is to improve (but maybe not perfect) the existing battery installation within its physical constraints and limited budget, as well as to understand the use of interconnects between parallel strings.

If I understand correctly, the interconnects between B+/F+, B-/G+ and D+/H+ are, in principal, a good idea that help with battery balancing? Are there drawbacks to leaving some or all of these 3 interconnects in place.

I ask because one of the ideas that I’m contemplating is to remove the second parallel string completely. In other words, use only batteries E, F, G and H if the drawbacks of the 3 interconnects are onerous. Obviously, I’ll have to see how much this energy storage reduction affects our ability to comfortably enjoy the RV, but it would be a simpler energy storage system…

All thoughts welcome.

Cheers.

4

If the interconnected can handle the full current then I suggest you leave them as normally they see little current. The advantage being that the 22V load is shared better and if you add a balanceruou only need 1. I would also monitor the 22V mid point voltage and consider if you need a balancer.

5

Thanks for sharing your thoughts.

I’ve got a BMV-702 and will be installing it in the coming days. Will look to set up the midpoint monitoring that this BMV is capable of.

The interconnect wiring between batteries in series or between batteries in adjacent banks is 1/0 fine strand welding cable. All lugs and battery posts have been cleaned of corrosion and all connections will be re-done. With the 1/0 and the renewed connections, I’m hoping that there will be lots of capacity to carry inter-battery currents.

Please feel free to share any additional ideas and thoughts.

Cheers,

/paul