First Battery Backup - Review Requested

erikm101 · December 2014

Hello!

I am new to the solar/wind/battery thing and would like to solicit criticism on my first battery backup design. I am primarily interested in feedback on things that might cause a catastrophic failure (think fire or explosion). I realize that a 12V system is pooh-poohed around here (based on what I have read in these forums so far), and I also realize that I do not have the best of components. That said, these are the components that I have at the current time and this is what I must work with for now. Please do let me know if you see anything dangerous, or just plain wrong.

The purpose of this system is to provide backup power in the event of a grid outage in my home. Loads would primarily be limited to some LED area lighting, mobile device charging and periodic running of refrigerator and freezer to maintain proper temperatures. Though I would like to, I do not intend to run the well pump at this time, as it is 220VAC and I lack the proper inverter for this. This is something I am looking into getting addressed.

The battery charger would be kept on and connected to the batteries at all times from grid power. During an extended outage, I would use a Ryobi 2200W inverter generator to power the battery charger. Same generator would likely be used for running higher draw loads like fridge and freezer while batteries are charging as well. I will likely only have one of the two inverters shown in the attached diagram connected at any time, based on the loads I determine I need. Not shown in the diagram, but I will likely switch the positive leg of each of these separately.

Questions that I have are:
-- Do I have the fusing right (single fuse on 12V + from bank to bus bar) and what size fuse should this be (I am guessing 250-300A)?

-- Since I am using the SmartGuage method 4 wiring, my individual draw at full load should be less than the load rating on the #2AWG wire, but am I correct? My math says 225A max continous draw over 4 batteries = max 56A per battery, well within limits for an 8-16" cable.

-- On the positive leg from bank to bus bar, will the switch and fuse significantly effect the resistance on that leg? I plan to keep the total wire length between all connections the same total length as negative leg.

I hope this information, along with the attached system diagram, provides enough information for meaningful discussion. If not, please ask whatever questions you may have and I will try to answer promptly. Thanks in advance for any assistance or advice you might be willing to give.

Erik

Attachment not found.

Note: I do intend to upgrade to a 24V or 48V system in the future that may be tied into my house wiring via a PSW inverter/charger system. Sadly, this is not within my budget at this time.

Cariboocoot · December 2014

Re: First Battery Backup - Review Requested

Welcome to the forum.

Looks like you've been doing proper research. The Smart Gauge wiring will help keep the batteries balanced, but as you can see it also explains one reason why 12 Volt systems are not that desirable; you've got four batteries in parallel with resulting possible parallel battery issues.

But it will work so let's move on.

One fuse: not enough.

The output of the charger needs a fuse to protect the wiring to it. That wire size must be at least 10AWG to handle 30 Amps, and the fuse has to be smaller. If the charger runs up to 30 Amps as it is likely to a 30 Amp fuse will pop sooner or later even though nothing is wrong. So this should be 8 AWG wire and a 50 Amp fuse between the charger and bus bar.

The inverter at 1600 Watts will draw 150 Amps or perhaps a bit more at maximum power/minimum battery Voltage. The maker should have recommended wiring and fuse size for this, but if they didn't you should expect to accommodate 175 Amps here. That would be 0 AWG & 175 Amp fuse/breaker.

The inverter at 750 Watts will draw about 70 Amps or perhaps a bit more. That would be 4 AWG & 100 Amp fuse/breaker.

Wiring from the batteries to the bus bar must be sized to meet the maximum current, which would be both inverters at once with no charge coming in or 220 Amps. That is 00 AWG and 250 Amp fuse/breaker.

In fact to be fully safe each one of those batteries should have its own terminal fuse. The question of wiring them together with small wire makes this mandatory: if for any reason you have failure of some of the wiring whatever is left has to take the full load. So if you count on dividing up roughly 200 Amps between 4 batteries and calculate wire size for 50 Amps each, then one of those wires/batteries fail then you have three carrying 200 Amps or 67 Amps each. Another one fails ... two carry 100 Amps each. So wiring that can and is expected to handle 50 Amps is now strained trying to handle 2X what was planned for.

Now 2 AWG can handle 100+ Amps, but ...

If you put 100 Amp terminal fuse on each battery if the current should go too high for the 2 AWG for whatever reason that fuse will blow and that part of the circuit is protected. The downside is that this usually results in a cascade failure of all of them. The way around that is to use all wiring that can handle the full amount and let the batteries take the strain if it happens (resulting lowering of Voltage should cause the inverters to shut down).

Switches and fuses rated for the current and Voltage use will have no noticeable affect on resistance in the wiring.

AuricTech · December 2014

Re: First Battery Backup - Review Requested

The biggest issue I see is that having four batteries wired in parallel (especially AGM batteries, since you can't check specific gravity of each cell to detect cells going bad) increases the risks of uneven charging and uneven discharging, neither of which is conducive to long battery life.

westbranch · December 2014

Re: First Battery Backup - Review Requested

Questions: is your fridge a new one? Do you have its consumption ?
I doubt small inverter will be able to run anything but an inverter type fridge due to the surge draw of most fridges. The larger on may work on the fridge, model dependent
How big is your house, if small you might consider 12v DC lighting rather than 120VAC, also DC for the recharging station...

hth

erikm101 · December 2014

Re: First Battery Backup - Review Requested

Thanks Cariboo for the fuse information. I will get those fuses in for the other electronics. To be clear, you recommend the 250A main fuse from bank to bus bar in addition to the individual fuses on the inverters and charger?

On the battery cables - if I understand you, you are saying that I either
A) use #2AWG and fuse each battery at 100A or

have no fuses and use 2/0AWG wiring?

erik

erikm101 · December 2014

Re: First Battery Backup - Review Requested

Hi westbranch. My firdge is newer (<2 years) but I do NOT have the consumption info on it. None of the manuals I have contain this data and I haven't gotten round to pulling it out of its cubby to see if it is on the back. I should be getting some diagnostic tools for Xmas though (Kill-A-Watt and a clamp ammeter) and can hopefully get real data in the next month or so. When run from battery, the fridge and freezer would be on the 1600W inverter with each having its own 24hr timer box and set to run for a set period, at different times. Probably on for 30-45 minutes every 3-4 hours and never both at the same time.

My home is 2000sqft. I t was pre-owned, so would be a pain to rewire for DC lighting. Long term plan is to have a whole home sized battery bank and split phase inverter (likely 48V) tied into the house wiring to avoid running extension cords during an outage. I am in process converting all lamps to LED for lower consumption, but it goes without saying that when running on battery we will have most things disconnected.

Cariboocoot · December 2014

Re: First Battery Backup - Review Requested

erikm101 wrote: »

Thanks Cariboo for the fuse information. I will get those fuses in for the other electronics. To be clear, you recommend the 250A main fuse from bank to bus bar in addition to the individual fuses on the inverters and charger?

On the battery cables - if I understand you, you are saying that I either
A) use #2AWG and fuse each battery at 100A or
have no fuses and use 2/0AWG wiring?

erik

If you use 2/0 AWG wiring you can use a single fuse on it rather than four. It is not ideal, but it will work.

It may help if you think of the system outlined as four separate circuits that interconnect:

1). Batteries to bus bars
2). Charger to bus bars
3). Bus bars to large inverter
4). Bus bars to small inverter

Each of these four will be subject to different maximum currents and must be wired and have over-current protection to handle that much current. The first one has the highest current potential because its maximum is the maximum of the two inverters combined.

westbranch · December 2014

Re: First Battery Backup - Review Requested

there are some threads here on fridges you might want to read.
IIRC the timer may increase your total load as the fridge will start-up with a defrost cycle each time power is 'lost' and that uses extra power... something to check with the manufacturer. Some inverter fridges have a much lower surge at start-up but which ones is a question...

hth

AuricTech · December 2014

Re: First Battery Backup - Review Requested

Given these two statements you made in your original post:

Your system, as posted, is based on components you already had on-hand
You plan to upgrade to a 24V or 48V system, when funds allow

I suggest that, at the time you make the upgrade, you also upgrade to pure sine wave inverters. While your refrigerator and freezer ought to function adequately on AC power from a modified sine wave inverter, the compressor motors will operate less efficiently on MSW power than they will on PSW power.

erikm101 · December 2014

Re: First Battery Backup - Review Requested

Thanks AuricTech. I do plan to use PSW inverters at upgrade time.

erikm101 · January 2015

Re: First Battery Backup - Review Requested

Another couple of questions:

1) Referencing the original drawing I posted, is it necessary to make the positive and negative wires from the bus bars to the components (charger, inverters) the same length? All battery interconnects are the same length and the cables from batteries to bus bars will be the same length, but what about the other components? If not, dressing in and tying down the wiring on the board will be much easier/neater.

2) I just got my hands on the 2/0 wire I needed to finish my installation. I did not realize until I got my hands on it that instead of a few dozen thick copper wires, it is made of something like 1200 very fine strands. Is there any downside to using this wiring for my batteries? I wouldn't think so, as it is still 2/0 awg and electrically should be the same, but I wanted to ask the experts. On the upside, it is quite flexible.

Thanks again for your time in answering a n00b's questions.

erik

inetdog · January 2015

Re: First Battery Backup - Review Requested

erikm101 wrote: »

Another couple of questions:

1) Referencing the original drawing I posted, is it necessary to make the positive and negative wires from the bus bars to the components (charger, inverters) the same length? All battery interconnects are the same length and the cables from batteries to bus bars will be the same length, but what about the other components? If not, dressing in and tying down the wiring on the board will be much easier/neater.

2) I just got my hands on the 2/0 wire I needed to finish my installation. I did not realize until I got my hands on it that instead of a few dozen thick copper wires, it is made of something like 1200 very fine strands. Is there any downside to using this wiring for my batteries? I wouldn't think so, as it is still 2/0 awg and electrically should be the same, but I wanted to ask the experts. On the upside, it is quite flexible.

Thanks again for your time in answering a n00b's questions.

erik

The fine stranded wire is not necessarily compatible with many crimp and screw terminals. In some cases all it takes is wrapping some thin copper around the strand bundle to make it work, but there are compression fittings that are specifically rated for both standard and fine strands.
Do not try to solder it to make it work.

Any time two wires are in parallel between the same two points they need to be the same length. And wires from multiple batteries to a common bus bar should be the same length. If you have two series strings of batteries, the jumpers between batteries should be the same length for the two strings.
But there is no need for the + and - wires to be the same length.

erikm101 · January 2015

Re: First Battery Backup - Review Requested

inetdog wrote: »

The fine stranded wire is not necessarily compatible with many crimp and screw terminals.

I am using PICO crimp lugs and they seem to be OK. I cannot pull a connector off by hand after crimping.

inetdog wrote: »

But there is no need for the + and - wires to be the same length.

That is excellent, and as I expected.

Thanks for your quick reply.

erik

First Battery Backup - Review Requested

Comments

Categories