I'm looking for a UPS solution, and think I have an idea that might work pretty well.  Looking for input as far as whether its a bad idea to do what i'm thinking or not.

 

First some background.  I have a decommissioned ambulance with solar panels on the roof, and 600AH worth of batteries.  It also has a tripp-lite inverter/charger.  Currently I have a heavy duty extension cord run to the house from the ambulance solar, and when power fails I can switch my medical equipment over to the battery bank.  

 

So, here is my idea.   Since the tripp-lite is an inverter/charger, I an run a second heavy duty cord back out to the ambulance and plug the inverter in to the house mains. (which would help ensure the batteries were always at full charge)  In this way, anything I plug in to the ambulance is running off of the mains, passed through the tripp-lite.  IF power fails, it will be auto switched to the battery bank.  

At this point, all I need to do is add a cheap apc ups and plug it into the wall somewhere, then connect it to my unRaid server. (I can have my router/etc hooked to the cheap ups as far as that goes) When power fails, the ups signals my server and it shuts down, and use the solar for my medical stuff.  

 

I have a call in to an electrical engineer friend who can hopefully tell me how dangerous this is. (2 extension cords, 2 charge sources to a really large battery bank.. He'll probably laugh at me)  Assuming the idea is at least feasable, and my friend can help ensure acceptable levels of safety, does anyone see any other potential issues I might face?  

Theory sounds great. Devil is in the details, but you said you've got a EE looking in to that.

 

As far as the danger of 2 charge sources, I can't really see any as long as the charger is designed for shore power + renewable.

 

If you are planning on staying in this location, I'd recommend setting up conduit with proper wiring to make the transition from inside to out, so you aren't running extension cords.

The ambulance itself needs to remain mobile, but I do like the idea of running proper wire in a conduit.  I can probably have a post placed that has the plugins I need so at least I can shorten the cord run. (similar to an  RV's setup, but with with both plugs, in and out, run to the same box on the post)  I'm now wondering if I can save a few pennies here and there by switching to the solar a few hours each day.  (could put a timer on the mains power.) I'll have to think on this.   Thanks for the input!

22 hours ago, sturgismike said:

I'm now wondering if I can save a few pennies here and there by switching to the solar a few hours each day.  (could put a timer on the mains power.)

If the dual source charger is designed well, it won't make much if any difference. When the solar input is above the threshold, it shouldn't draw any wall current at all beyond a status trickle unless battery charge demand is higher than what the solar can provide.

 

However... I can see a situation where offset demand times may let you see some savings, by turning off mains power at bedtime, and allow the batteries to remain somewhat discharged until the sun can recharge them. Actually, I kind of like that idea, but reverse of what you proposed. As long as the battery bank is monitored and not allowed to drop too low, I'd see about ONLY running mains power during the late day when the solar is mostly done, to let the mains top off the bank during peak evening draw. Only run the mains for enough time to supplement the solar to keep the batteries in the correct charge state.

 

If the batteries are always completely topped off by the mains at the start of each solar day, you aren't going to get ANY solar savings because the solar won't have anything to charge.

 

It's a very complex equation of minimum desired runtime while maintaining a battery friendly charge level.

 

Do you know how much KWH you use on average each day? Do you know how many KWH per day you need to survive if the ambulance batteries are your only source? Do you know how much KWH the solar panels can produce per day? Do you know how many usable KWH the batteries hold when charged fully and discharged only to a healthy level? Do you keep the ambulance filled with fuel, and can the engine powered charge system keep up with your demand?

 

That system is a really neat way of keeping power available at all times, and with the possibility of triple source power (solar, mains, fossil fuel) I would not be scared to try and optimize your solar gain by actively managing the battery bank charge state. Maybe even add a few solar panels on fixed mounts beside it if the charge controller can handle the extra power. You already have 90% of the infrastructure, why not try to optimize it?

You're right, it is pretty complex.  I'm going to have to think about this a bit more.

 

One of my concerns with the dual charging is that both the charger, and the solar charge controller adjust based on sensed voltage, and I read somewhere (I'll have to find it again) that this can cause a type of oscillation, where both sources spike/drop/spike/drop in an apposing pattern based on what the OTHER charger is doing.   

 

Lets see.  First, if I drop to bare minimum usage (medical equipment only,) and if days are sunny (pretty likely where I live) I can run my medical stuff about 18 hours a day for about 3 weeks. If I were to add 1 more panel, I should cross the threshold of indefinate running.  My current equipment can handle another panel, and yes, that is in the works.  To really improve the system, I'd need to switch my battery bank to 24 volts allowing me to run twice the panels with the same charge controller, but then that would require an investment in a 24v inverter.  Might be worthwhile, since it would remove my need to have extra charging capacity (so I could run more equipment for longer.) 

 

As far whether turning the charge off during the day vs the night.. If I understand correctly, placing a draw on the system during the day while the sun is out will draw from the panels even if the batteries are topped. Running the charger during the day would mean that a) I'd be running off of my mains, and b) the charger would be doing most of the work topping off the batteries. (IE wasted sunlight)  Ignoring the whole "will it oscillate" issue.  My personal thought is that a combination of the 2 options would be best.  Turn on mains/charging in the evening when the panels can no longer keep up, minimizing battery wear and tear, while ensuring they stay close enough to full charge in case of power emergencies.  Then, figure out WHEN to turn the mains back off based on all the things you pointed out.  My usage, solar generation capabilities, and whether I can end the most productive part of the day with full or nearly full batteries.  

 

Thank you very much for the thoughts, I'm now officially in the "add more panels" camp.  Add enough panels and the mains charging ability becomes much less of an issue. (though it would still be nice to have a way to auto swap from  mains to battery and back, but I can probably pick up a simple relay of some type for that.  

1 hour ago, sturgismike said:

One of my concerns with the dual charging is that both the charger, and the solar charge controller adjust based on sensed voltage, and I read somewhere (I'll have to find it again) that this can cause a type of oscillation, where both sources spike/drop/spike/drop in an apposing pattern based on what the OTHER charger is doing.   

Oh... That changes things. I ASSumed you had an intelligent dual source solar charge controller that used mains internally. Yeah, you really don't want 2 independent chargers on the same battery bank at the same time. Ideally you would have only one charger intelligently managing the state of the bank 100% of the time.

 

In that case, yes,

1 hour ago, sturgismike said:

Turn on mains/charging in the evening when the panels can no longer keep up, minimizing battery wear and tear, while ensuring they stay close enough to full charge in case of power emergencies.  Then, figure out WHEN to turn the mains back off based on all the things you pointed out.  My usage, solar generation capabilities, and whether I can end the most productive part of the day with full or nearly full batteries.

 

I know power emergencies are your real concern, but since you can always fire up the ICE in the ambulance and burn fossil fuel, maybe keeping the batteries fully topped isn't the most efficient strategy. In a real power out for days scenario, aren't you going to relocate and fire up the ambulance anyways? If your specific geographic location is going to be out of power for longer than your batteries and solar can hold out, you are probably going to need to drive at least an hour away to get to an area with food, fuel and other support.

 

Your situation seems to be divided into 2 pretty much opposite conditions. Stay put with mains available, maximize efficiency and minimize cash outlay, or, bug out and get to an area that allows you to get back to condition normal. The transition time between the 2 conditions is when you are the most vulnerable to power deficiencies, and you can always idle the ambulance to make up the power shortfall.

 

1 hour ago, sturgismike said:

To really improve the system, I'd need to switch my battery bank to 24 volts allowing me to run twice the panels with the same charge controller, but then that would require an investment in a 24v inverter.

 

If you do this, don't just go to 24, step up to 48 or more. Better panel options, and just better equipment in general is available for similar costs. The rise in home solar popularity has really helped with economies of scale here. Just keep in mind you will still need to have a 12V power source input option available so you can be mobile and run off the standard alternator.

I think you hit it on the nose.  For short term (1 day maybe 2) emergency power, I'm set.  Longer than that, and I should indeed skedaddle.  I had intended  setting the ambulance up as a camper for long term, middle of nowhere camping but not long after I bought it and started the project my health took a major turn to the point where I can't even get into the darn thing now.  (I now have a van with a wheelchair lift to make me mobile again)  

 

ITs too bad, the ambulance is nothing short of awesome.  (diesel, electrical already run, so adding the solar was easy enough, tons of storage, already has cabinets etc built in)  Perhaps it'd make sense to shift the solar to the van so that I can get back on track as far as camping goes. Not nearly as much space for panels though.  (there is room for about 6 more panels on the ambulance roof)  

 

Time to consider a "for real" solar system now I guess. (grid tie/hybrid) so that I still have my battery bank for emergencies.  

 

Guess its time also to think about selling the ambulance.  No sense having a vehicle I can't drive any more.  

 

Thanks very much for all the input.  It always helps to figure out a plan of action prior to diving in.  You've been lots of help! 

22 minutes ago, sturgismike said:

I had intended  setting the ambulance up as a camper for long term, middle of nowhere camping but not long after I bought it and started the project my health took a major turn to the point where I can't even get into the darn thing now.  (I now have a van with a wheelchair lift to make me mobile again)  

Hmm... maybe investigate what it would take to get the ambulance usable for you? That is indeed an ideal platform, the mechanicals are so much better than a standard van. Plus, people get out of your way when you drive it.

 

If you can spend a few grand to make it usable, that would be ideal.

Lots of people manage to adjust their standard-sized cars so they can make use of them despite having a wheel chair. But I guess they often rely on someone to help them stove the chair after they have gotten into the car. I only think I have seen a van with lift equipment to load/unload the chair allowing the user to manage everything alone.

 

When it comes to panels, you can always make use of fold-out solar panels since you wouldn't need them when driving.

 

And as noted - you want as high voltage as possible since that keeps down the currents, losses etc. It's way easier to handle 200V 10A than to handle 12V 150A.

Thank you both!  

On the subject of the ambulance, cost would be prohibitive.  The pass through to the drivers side is small, so I'd have to figure out how to get to and from the drivers seat, and then I'd have to have a lift to get the chair in and out.  My f250 van has a lift and a 6 way seat making transfers easy.  Though I don't need them yet, it also has hand controls.  The van is old, but it works far better than the car I had that I could no longer use.  I had reached the point where I only got out of the house about 3 times in a 6 month period.  Not a fun situation. (thankfully I have good people in my life that help out)

 

We're usually pretty good at finding answers to problems. The most recent being an inability to do tub transfers. (yes we're getting WAY off the unRaid topic here..)  After researching lifts/tracks etc through medical supply shops, the answer I would need to solve the problem was going to run around 6k before install costs. (And no, they wont just sell you parts, they insist on installing.)     Instead, we rolled our own solution.  A small ibeam track, heavy duty trolly, and hoist, with a harness I made myself.  Installed the thing ended up costing around 800 for everything, with the added benefit that if I need to, I can change out a car engine in my bathroom if need be. In fact, I used the lift to get my z800 from floor level onto a spare wheeled walker, then used the walker to get the thing into its final position. 

 

While I wouldn't call it "fun", but knowing where i'm headed as far as ability goes, it is definitely interesting to plan ahead as much as I can so that I can manage as much as possible on my own for as long as possible. (on my own, but with help to figure things out and implement solutions)  

I'm attaching a picture of our home grown ceiling lift and the ambulance, in case anyone is curious.  

 

Again, thanks to you both.  Much to ponder now.