A glimpse at my back-up power system

[vtpeaknik]

In the last few months I've been building a back-up power system.  The system evolves in my mind as I work on it, so it's a moving target to some extent.  The original idea was a smallish (1 KWH) battery setup plus a medium inverter, charged from the grid.  I was just hoping to run a small freezer and a couple of lights for a limited time in case of power outage.  By now it's evolved into a 5KWH (nominal) battery bank (12V 450 AH - 4x golf-cart size Rolls), a sizable inverter/charger (used Prosine 2.0), 360W peak PV (2x SunElec 180), MPPT charge controller (blem BZ), etc.  Now thinking of adding another small chest freezer in the cool basement, configured with an external thermostat to run as a very low-energy refrigerator.  I've run some AC cabling from the inverter output to 2 outlets, one central in the house, one out in the mud room.  Also some 12VDC outlets.

Here are a few photos of my homebrew electrical panel for the system.  It's now complete except for the PV feed cables, which will attach to the charge controller in the upper right corner.  The second charge controller under the main one is for auxillary PV panels if I ever feel the need to set then up in the yard if I'm desparate for every last bit of juice - I have the HF 45W set stored as a spare...  The DC (and AC) protection is all breakers.  The AC sockets on the left are the grid input, the inverter/charger AC input (orange plug, NEMA 20A plug to prevent plugging the inverter into its own output) is seen plugged into an adapter to 15A plug for connecting to a generator if needed.  Batteries are in a plastic storage container (with flat bottom, hard to find).  Vent pipe has a small fan in the top end (exhaust), the circuit to turn it on when the batteries hit about 13.8V hides in the switchbox at the top (to the left of the charge controller) - the switch is for manual operation of the fan for the purpose of airing out the basement.

[strider3700]

I've been thinking of putting something similar together.  

Does the inverter handle the battery charging?  

My goal long term was eventually to have battery backup when it's needed and to keep the batteries charged via solar/wind and grid if need be. Put a few of my constant loads (PC, water system...) on the system and do my best to get them off grid. Somehow set it up so grid only kicks on if the batteries are drained too far.

[Volvo farmer]

That is a very nice system. I like how you custom built that enclosure for the DC breakers. Everything looks like top-quality components and done up right.

I have an axe to grind with BZ products, and here I find another opportunity to publicly bash them :-)

Be very very careful with that charge controller. If it is a MPPT500, it has only one setting, float. Other charge controller manufactures have known for years that there is a better way to charge batteries, it it a three stage algorithm that consists of bulk, absorption, and float. Factory float setting is 14.1 on a MPPT500. That is WAY too high a float setting, check your Rolls recommended float, I bet it's no higher than 13.6. This is especially problematic in a system that is used mostly for backup as the batteries boil all day long every day and use lots of water.

Unfortunately it is also problematic in a system that gets used relatively heavily The charge voltage for those wonderful batteries you bought should be higher than 14.1 for an hour or two each day, at least 14.4 and as high as 14.8, I'd bet. Then the voltage is supposed to be reduced to 13.6 or so so the batteries don't use so much water.

I had a BZ controller on my seldom used system that is very similar to yours. Batteries would show the tops of the plates in 60-90 days. Luckily, my controller quit working in 18 months and I got an old school Xantrex c-35. No MPPT on the Xantrex but it works on the proper algorithm, floats my batteries at the proper voltage and does not boil all the water out of my batteries.

I feel that BZ uses the MPPT buzz-word to promote their products as "better" than the competition, when in fact they don't use the three stage charging process. They got me when I was new and uninformed, then they got me again when my controller failed and they wanted ten bucks to ship a replacement back. The busted thing is still in my closet.

Thanks for listening and good luck :-)

[vtpeaknik]

Thanks for your kind words on my wiring job.  I tried to do it right, although many of the components are from the local salvage place.  (Not the DC breakers, those I had to mail-order from an alt-energy dealer.)  I still need to label everything, and write up instructions in case somebody else in the household needs to use the system while I'm away...

My PV panels are rated 25V at max power, and I'll have them in series, i.e. 50V at max power.  This allows a much thinner cable.  But requires a voltage-converting controller, thus I can't use an "old school" one.  If I end up abandoning the BZ, I'd have to get a much more expensive one, e.g. "Outback".  Hopefully the BZ will do the job.  Perhaps some day they'll update the software to allow the battery voltage to rise over 14V and then float back down at 13.something.  (Not sure if that would only require a software change.)

But with the float voltage set at about 14, and full sun only a fwe hours every other day say, would that still "boil" the batteries?  And if I add water as needed every few weeks, would that be a problem?

[Volvo farmer]

You've got 360W of panels right? What's you're wire length one way from them to the batteries and what is the size (in AWG if you can)?

Here's what I think.. If you are truly using this as a backup system, and are not loading the batteries except for emergencies, reduce the float voltage on the BZ to 13.8 or 13.6 and call it good. This will reduce water consumption, and maybe reduce plate degradation as well. Sulfation shouldn't be an issue as the batteries are not in regular discharge use. BZ has no software in the MPPT500 that can be changed, trust me.

If you have regular, daily loads on the system, try and get a three stage MPPT controller. Morningstar makes one as well as Outback. With only 360W of panels though, maybe a little power loss in the cabling is worth the tradeoff in a $90 charge controller. It would depend a lot on your daily electrical requirements from the system.

Let me tell you, I ran my whole, four person household on a system that was only slightly twice as big as yours, with very little generator backup, for over a year. No electric fridge or freezer then but I think that system you built is very viable for long term emergency use or even daily single room or single use loads.

Congratulations in making your system scalable. I believe you have chosen some of the basic building blocks there that can take a whole family off the electric grid if you choose (or need) to do so. I see true deep cycle batteries, high quality cabling, a good SW inverter with charger built in, DC breakers everywhere they should be... etc.

Not trying to inflate your ego but I think this is a pretty nice, easilly scalable micro PV system.. Good work.