Connecting batteries in Series and Parallel

[BobG]

[BobG]

I am really uncomfortable about what I would like to do. A lack of understanding possible pitfalls and events has put me in this situation. I have the batteries.

The objective is to operate an air conditioner intermittently for short periods at bed time to cool the trailer down without having to carry a generator or connect to shore power.

To do so, I decided to get two 12 volt batteries and wire them in series to connect 24 volts to a 3,000 watt inverter. My thinking was that if necessary, I could get 2 more of the same batteries and then connect the 2 series groups in parallel for more capacity.

Part of my problem is that I can only connect 1 conductor to each battery terminal due to the batteries using Anderson connectors. The connectors will be pretty much maxed out with one 1/0 cable which is the recommended size for the inverter.

This is a sketch of what I am planning if I need to add 2 more batteries. What am I doing wrong or what problem will I have in keeping the batteries evenly charged. Solar will be primary source of charging.

[wackyburggeezer]

If you use either of the two sketches, you'll also need to change your trailer lighting, refrigerator, electrical brake actuators, and a few other items.

[Greg A]

I’m not following ur reason for wanting to step up to 24v and then have to step back down to 12v to operate the trailer. What is the advantage?

[BobG]

My problem is that to operate at 12 volts requires 250 amp connection to the inverter and that requires 2x 1/0 conductors per the inverter manufacturer's (Victron) recommendations.



The battery connections are Anderson SB175 connectors and they can only accept 1x 1/0 conductor.



I can connect 2 12v batteries in series for 24 volts and then 125 amp current can be supplied to the inverter with 1x 1/0 conductor each way which complies with the recommendations for the inverter.


In case I do not have enough capacity to run the air conditioner as I would like, I have room to add 2 more identical batteries but then I need to connect the 2 serial groups in parallel for what I am calling a 2s2p configuration and I am not sure I am doing that correctly.



My concern is that the only examples I have found have connections made directly to the battery terminals and also I think I saw one example where there was a cross connection, that I am not showing, connecting the + & - connection in each group to each other. That is where things get beyond my old brain. I understand the need to match the length of conductors to even out the charging but I am not sure that that alone will do the trick due to variances in the connections, especially the Anderson connectors. That is where I think the cross connection was made in the example I saw. I may try again to find it. I may be all wrong in my recollection.

[HABBERDABBER]

My response does not further your battery questions, but comparing weights of the multitude of batteries, would it not be advantageous to just use a small gas/propane powered generator to do your short term cooling?

[BobG]

I have considered a generator but it requires gas unless converted to propane. If I get one I would convert it to propane. The problem I have with the generator is that the only place for me to store it out of site is under the bed and that isn't easy. The Honda 2200i won't fit through the access hatches to storage areas. I have thought about moving the spare tire and adding a box to the rear bumper for the generator but I would just as soon not even deal with one and the fuel it needs, whether gas or propane.


The 2 batteries I have are 46 lbs each so 91 lbs total for 170 Ah at 24 volts or 340 Ah at 12 volts. I do not know what the Escape batteries weigh or how many usable amp-hours are available but the weight has to be close to a trade off. Hopefully I will be able to cool the trailer in 15 minutes at bed time and then maybe another 15 minutes a half hour later. This worked for me last year but I plugged in.



Hopefully I don't have to add 2 more batteries but just in case, I want to know that I can do it if necessary.

[Eggscape]

Don’t get me wrong as I am all in when it comes to Anderson Power Pole Connectors.
I have them all over the trailer and it is my choice for the low draw I have on my batteries.
However in your case can’t you just use the right size wire at 12 volts and a wing nut or proper nut on the batteries when you want to remove them. For high amp stuff I always like to avoid as many connectors as possible.
Or
Use the sb350 rated at 350 amps as I do on my portable inverter.

[Eggscape]

I guess you will not be towing with a pickup because it is so easy to lock a generator to a siding tray like I made.
Works great to cool the trailer down for a quick mid day lunch on the road or like you say, to cool the trailer for the night.
Does dual duty as backup power at home as well.

[BobG]

Eggscape,


The batteries come with built in SB175 connectors. I didn't mention that the batteries internal circuitry uses 2 AWG which might be a little light for what the manufacturer says is continuous current of 175 amps.



If I ran the batteries in parallel as 12 volts I could definately power the inverter but it would require larger conductors that I can not connect to the built in connectors on the batteries.



It is not feasible for me to rebuild the batteries plus the 10 year warranty would be void. Says so right on the battery if I break the seal. I might be able to return them and buy different batteries but I am thinking that what I am doing is ok. They fit the area under the dinette bench just fine. I just don't know if I can properly configuring them for 2s2p if it turns out I need more capacity.



I honestly think I will be ok on capacity with 2x 175 Ah batteries but maybe not.


I tried crimping some SB175 connectors with 1/0 conductor using a hydraulic crimper and I tried a local outfit. Not very good results. I will need to have the cables made to order and am thinking of BatteryCableUSA. Do you know of other places to source cables with Anderson connectors. I am in Seattle area and have not found anyone able to do it. I know there must be a place but I have not found it yet. I am still looking.

[BobG]

Eggscape,


I don't have a pickup but if I get desperate I may decide to trade in for one. It will be hard to do. I don't really want 2 vehicles.

[Vermilye]

I don't see any problem with your series/parallel combination, although without seeing the actual specifications on the batteries, unless my math is wrong or you are looking at lithium technology, they seem a bit light at .27 amp hours per pound compared to .59 amp hours per pound for the Escape provided 6V batteries.

An equivalent AGM pair of 6V or single 12V would be about the same with, for example a Lifeline GPL-8DL AGM running around .61 amp hours per pound.

If they are lithium, check with the manufacturer to be sure the series/parallel combination as well as the maximum current draw you expect is allowed.

[BobG]

Jon,



https://bigbattery.com/product/12v-1...ck-rv-battery/



Their webpage says up to 8 parallel and 2 series for 24 volts. They sell them under the heading RV Batteries. They don't talk about paralleling 2 series groups. I will check with them tomorrow. I watched a video of one of these batteries being torn down and it looked decent on the inside. Time will tell. I am going to look for the schematic I saw one time of batteries being wired in series and then parallel. The way I remember it the wiring was different. Maybe I remember it wrong but what I am doing is similar to what I would do with solar panels. I know they aren't the same because the batteries need to keep the voltage of individual cells synchronized.


Bob

[rubicon327]

Just brainstorming...would a Victron Lynx battery combiner work? You can land four 12V batteries on the bussbar individually and keep them in parallel to maintain a 12V system. Or combine two 12V batteries in series first and land two sets on the bussbar in parallel for 24V. All battery positives would be fused. If you stay 12V you might be able to land your wires feeding the power center DC board on the posts on the one side and your inverter on the large lugs on the other side without any additional equipment. I’m no electrical expert...maybe one of the gurus will weigh in on this idea.

https://amsolar.com/victron/battbms-vtlynx

[Vermilye]

As long as you come up with a solid (electrically & mechanically) method of combining the parallels, it looks like it will work.

I wonder how they produce 12.8V - most lithium batteries use 4 3.6V cells per battery to produce 14.4V.

You can add a DC to DC converter to supply 12V for the rest of the RV appliances.

The Lynx combiner that Dave (rubicon327) shows would work to let you stay at 12V, while going to 24V & a DC to DC converter for 12V wold let you use smaller wire to the inverter.

[BobG]

Rubicon, I was thinking about trying to use some sort of heavy duty copper buss bar. I even looked up specs for them and tried to size one. That might be a good idea. I did take electrical engineering classes but that was a long long time ago. I went a different direction and it was probably a good thing. I am not good at dealing with electromagnetic things.



Jon, the webpage is know to have mistakes on it. The company is quick to make it right if a misprint by marketing causes a problem for anyone. When the batteries arrived, I checked one of them and it is sitting at 13.15 volts which I think is a happy voltage for sitting on the shelf.


Bob

[Eggscape]

My mistake...I missed that it had a built in sb175.

I don’t know of anyone that sell cables with the Anderson pin attached but our local battery shop will put almost any correctly sized lug onto a cable...for a price. I would think if you took the parts to a battery specialty shop that they would do it for you.

[Brian B-P]

Originally Posted by Vermilye

I wonder how they produce 12.8V - most lithium batteries use 4 3.6V cells per battery to produce 14.4V.

The spec sheet says that these are LiFePO4, and that the charging voltage is 14.6 V. That will certainly be 4 cells in series, and the 14.6V will be the design maximum (3.65 V/cell), but the nominal voltage will be the usual 3.2 V/cell... so 12.8V nominal. The spec sheet strangely gives the voltage range as 8.5 to 15 volts; the 15V is fine for (over)charged, but 8.5 V is only 2.125 V/cell, which seems alarming low.

[HABBERDABBER]

"I have considered a generator but it requires gas unless converted to propane. If I get one I would convert it to propane. The problem I have with the generator is that the only place for me to store it out of site is under the bed and that isn't easy. The Honda 2200i won't fit through the access hatches to storage areas. I have thought about moving the spare tire and adding a box to the rear bumper for the generator but I would just as soon not even deal with one and the fuel it needs, whether gas or propane."




Your vehicle towing your rig uses gasoline, or possibly propane, like any internal combustion device or generator. I do not completely understand your reasoning. Batteries (lead acid) are also hazardous items, producing potentially explosive hydrogen gas and filled to a brim with a very, very strong Sulfuric acid.



Gasoline, pound for pound, contains 10 TIMES (an order of magnitude, or 1,000% MORE) energy than a pound of batteries. A thousand %. Yes, that does not include the weight of the generator, or batteries, but that is a minimal variable.



So where's the trade-off and ease of use advantage of a battery?


One can do most anything these days....just get out your credit

card. Heck, we did that 50+ years ago and bought our way to the MOON.


P.S. I have a 19' rig and there likely ain't no way in Hades you'd get a gen set under the bed, unless a larger hatch, much larger hatch, was installed.


A rear bumper mounted or front A-frame stand-off mounted generator would be more practical in my view in your application. You certainly can go a bit exotic with a battery system, but if it's lead acid, that's not exactly an advanced system. AC's require a whole lot of amps to operate, especially at start up.


So if you can do indeed run your AC off of your batteries, how do you intend to recharge them for their next use, the following night?


P.S. In your calculations, remember that you have ONLY 50% (1/2) of the rated capacity of your lead acid batteries. To go below that amp hours rating at either 12 or 24 V is very severe duty service on the batteries and shortens their life significantly.

[richm]

Originally Posted by BobG

To do so, I decided to get two 12 volt batteries and wire them in series to connect 24 volts to a 3,000 watt inverter. My thinking was that if necessary, I could get 2 more of the same batteries and then connect the 2 series groups in parallel for more capacity.

Adding batteries later is much easier with lithium because the capacity degrades much slower. Lead acid batteries degrade, and as a result, the newer batteries might be supplying more current than the old ones, and be harder to balance.



If the batteries are well matched, then half of the power (1500 Watts) is delivered by each battery. However, an intermittent disconnection of one battery, would load the other with 3000 watts. So it'll be best to fuse each one at whatever the wire is rated for.

3000W / 12V = 250A. You also want some margin to avoid heat, etc. If the wire is rated for 250A continuous, you don't need a fuse. If it's less, then add a fuse for something at or below the continuous Amps rating.

Originally Posted by BobG

Part of my problem is that I can only connect 1 conductor to each battery terminal due to the batteries using Anderson connectors. The connectors will be pretty much maxed out with one 1/0 cable which is the recommended size for the inverter.

Where the leads from the two batteries meet, you have the full 3000 Watts, so you need a wire at that point and further that can carry 250A continuous. You'll need that size going to the inverter. A bus bar is a good way to connect lots of large wires together.



A Fuse box will combine the fuses, bus bars and a cover. The cover is helpful in terms of preventing things from falling on the bus bar.

Originally Posted by BobG

This is a sketch of what I am planning if I need to add 2 more batteries. What am I doing wrong or what problem will I have in keeping the batteries evenly charged. Solar will be primary source of charging.

It is far more common to first wire pairs of batteries in parallel, and then connect the parallel pairs in series. That way, you can use a just one BMS to balance all of the batteries.