solar/ biodiesel powered cinema

[kita]

lots of other examples of mobile solar powered entertainment exist in the UK (solarcinema, groovymovie, solar circus etc) but we wanted one of our own. The set up consists of a '98 reg mercedes 312 sprinter, with a digital projector and sound system pointing out of the back. An awning with detachable sides runs away from the back of the vehicle, like a 30 foot long corridor, to the screen. During showings the peak demand is about 1.5 kw, and they will last for an average of four hours, and showings will be on average once a week (only during the summer). Roughly then we are at looking at a 6kwh demand plus losses- too much in my opinion to justify a dedicated array given the infrequency of use. I have tried to minimise the quantity of panels required by allowing for a six day recharge time (20hrs sun x 300w panels), and incorporating a connection to a split charging system run off the vehicle's alternator (it is an ex-ambulance, running on biodiesel).

This is how I have designed the power supply-

two 24v 165w polycrstaline solar world panels (£600 each)

one 12-24v 45a steca tarom charge controller

one 24v 2.3kw struder inverter and battery charger

four 200ah 12v victron gel batteries (connected for 24v, £200 each)

My main questions are:

1.     If we were using the sound system as a small P.A., then we could see the inverter running continuously at 1.5kw at 240v ac (equivalent to 62A at 24v) Will this significantly shorten the life of the 800ah battery bank? The inverter is rated to 2.3 Kw continuous and 2.6kw (108A at 24v) for 30 minutes- how can I estimate the effect of this sort of loading on the batteries (over 10% rate of discharge compared to size of battery bank)? The batteries I have chosen are gels for maximum cycling, but would agm batts be a better choice because of the occasional high discharges? I can't see the spec sheets on the manufacturers site because it is in photoshop .psd format: http://www.victronenergy.com/product.php?productid=173. I was previously looking at sonnenschein/ trojan 200ah but they take weeks to order and are 25% more expensive.
   
2.     I have an isolator between the panels and the charge controller. The electrics on the vehicle incorporate a split charging system where an auxiliary battery is charged independently of the starting battery. I do not yet know what the open circuit voltage of the alternator is, nor its maximum amperage- it is not mentioned in the manual and I am wary of using my cheap multimeter!
   
   

What I would like to do is insert a two way sitch in the lines that run to the auxiliary battery, and divert it to the charge controller for the battery bank that supplies the projector and sound system. Provided the voltage is suffienctly high, and the amps are lower than 45 (the rating of the charge controller) can anyone see any problems with this set up? What would be the cheapest and safest way of doubling the voltage from 12v to 24v if that were necessary? Dc to dc step up transformers are expensive at these currents, and I have heard of some sort of simple 'switch-pole' system- any ideas?

thanks for any help, and please contact me if anyone is interested in contributing films, offering venues, or just seeing it in action

[maker of toys]

running a flooded cell deepcycle battery at 10% DOD per hour doesn't present too much trouble, as long as you dont goo too deep too often. the gel cells might be a little more sensitive, but i dont think a c10 rate is too much. (at the 62A rate you are probably ok for up to 4 hours.  you will want to stay away from the c8  or higher rate as a matter of course, but the occational sprint wont hurt too much. . . . the electric car guys  usually design for c5 with sprints to c3.

most automotive alternators have a VERY HIGH open circuit voltage, on the order of 200V or more. that sort of voltage can make elecronics more than a bit unhappy, as Im sure you realise.

A diesel engine alternator will likely be a higher current output unit than a gasoline engine alternator.  this will be especaially true of an ambulance. To get the exact specs, get the number off the alternator itself and have a automotive electrics outfit look it up for you. . . they might know just based on the vehicle information. some auto parts houses also have the information you seek in a catalog. . . . the sort of place that keeps bankers hours will be your best bet. . . the discount chains usually have discount help.

below, Im assuming a 12V starting system with 24V deepcycle bank. . .  

if you have an external voltage regulator, you can buy a second, 24V regulator and swap back and forth with a suitable relay and switch setup.  Remember to open the field connection before changing battery banks, so you dont get into open circuit voltage troubles. some sort of interlock to keep the batteries separate and the voltage regulators associated with the correct batteries would be a VERY GOOD IDEA.

if the regulator is internal, then you can still do this, but now you are in the realm of having to invent something. . . . OZtules posted a neat circuit a little while ago that could be modified.  the usual disclaimers apply. . .

 the biggest factor in getting a higher voltage is the diode bridges in the alternator, which are sometimes a piddly 50piv or so for US models. the electric shop should be able to tell you exactly.  other than that, it is all a case of how much wiring you want to do.

OR

you could buy a fair-size battery charger, plug it into the inverter, and use that to charge the starting battery. . . run everything else off the 24 V side. . .  this will, of course, require you to determine what constant load is on the 12v system as far as engine management computers, fans, lighting, etc. (24V headlights, tail lights, dome lights, etc. are available. . . .

Dan

[altosack]

Hello kita,

1. Yes, you will shorten the life of your batteries by draining them by a continuous 62 amps (more like 70+ if you take into account the inverter efficiency) with 400 Ah batteries (if it is connected at 24V it is 400 Ah, this is a discharge rate of C/6 !).

It will also reduce the capacity of your batteries; the total will be less than 400 Ah at C/6 (batteries are usually rated at C/20 or C/100 for deep cycle use). I would expect them to de-rate to something like 280-290 Ah at this discharge rate.

You mentioned that you're using gels instead of agms for better cycling; well, neither are very good for deep cycling. I would use flooded lead acid, which are much better at deep cycling, will tolerate higher discharge rates, and cost much less, to boot. I think over the long run, it will be worth it to solve the problems of gassing and jostling (I assume these are the reasons you're using sealed batteries). This would also allow you to afford more, so you could get at least in the area of C/12; C/16+ would be better for long life of your batteries.

2. I'm not sure how you're going to get your vehicle's charging system to work this way. It is set to regulate at about 13.8V, so in order to use your solar charge controller, you will have to either disable the regulation temporarily (risky, in my view for a vehicle you also want to drive) or put in a DC-DC converter that maintains the input voltage at less than that to get full charging current.

There are voltage converters commercially available, but as you've seen, not at this amp rating, at least not for a reasonable price that I am aware of. If you are handy, you could build your own boost converter with a microcontroller and PWM; there are some ideas for that on this board, but not with enough details for you to finish the project unless you really know what you're doing.

Why don't you just use 12V for your system (ohmigod, did I just say that ? I never say that !) ?  I realize that it's less efficient wire-wize, but a few bucks for thicker wire (say 4/0 for your inverter and 4 ga from the alternator and 8 ga from your panels to reduce losses) would be much less than other alternatives, especially since the wire runs are not that long. Although vehicle alternators are not that good for charging deep-cycle batteries because they regulate at about 13.8V, which is too low, you can always top off with your solar panels.

Good luck,

Dave

[maker of toys]

I seem to have bobbled the math.  I was thinking of 800 AH.  using 4oo AH, 62A is getting into the danger zone, and the 108A sprints will be abusive.

also, with 400 AH  of gels to play with, 2 hours is about all Id try for at 62 amps. . . better to use less DOD if you can.

Dan

[kita]

Firstly, thank you for your thoughtful and detailed replies.

The alternator spec investigation is in progress and the results should be through on Monday. I have decided to steer clear of any modification of the alternator itself or its regulator as all the vehicle electrics are 12V. I also don't have the time or expertise for a home built 100A dc-dc converter. As we said commercial versions do exist from Studer and Vanner, but they are only rated at 10A 12v-24V. So-

1. Back to 12V? I am considering this, and the main issue is the lead time and delivery from my suppliers (I can't wait another 2 weeks for 12v panels). We are also talking some very substantial wires for 220A at 12V (inverter peak). I would have to parallel two 50mmsquare multistrand copper cables- any ideas on how best to attach two cables of this size to a single battery post? I could drill out copper pipe for the batt terminal connections, and just use cable for the inverter connection, but I don't know if even that would be beefy enough.
   
2. Use a chageover switch to change the battery bank voltage from 24V for solar and mains charging, and inverter use, to 12V for alternator charging. Downside is I cannot alternator charge and use the inverter at the same time, but as it may be my only option I am looking in to it.
   
   

I would first have isolators on the runs to the inverter and solar panels. A 150A rating at 12v would be fine for 110A at 24V (inverter peak) right? I might bypass the charge controller completely as I think its 45A rating will be too close to the alternator charging current, and the vehicle already has a built in regulated charging system that will be fine for flooded batts (the vehicle's auxiliary battery is a gel anyway).

Then I would use a product like this from vehicle-wiring-products.co.uk:

 Marine battery change over switch.

Phenolic moulding with solid copper contacts.

175A at 12v, 300A for 5 seconds.

Selects between batteries; one, two, both or off.

10mm stud connections.

(http://www.vehicle-wiring-products.co.uk/VWPweb2000/photos/60510.jpg)

The main thing I am not sure about is the wiring for such a set-up. Thanks to your advice, and a contact from Hugh Pigott, I have decided to go for 6x235Ah flooded 12V batts. Semi-traction monoblocks, with catalytic caps. They will be in a sealed box, with hinged lid, and external ventilation, mounted at right angles to the chassis to balance the weight. I was thinking of having two banks of 3x12V batts in parallel, and then series the two banks for 24V. I can't figure out how to switch back and forth between a 12 and 24V configuration without using two changeover switches, and seperate cables at the two crossover points, one for series, one for parallel. I hope you follow me, but as it is I am getting a little confused...

I also don't know what effect this will have on the balance of the batteries, or if the switching could damage them. There is an equalising charge mode on the inverter-charger, but would I still have to rotate the batteries to prevent the end ones wearing out?

3.I have a similar wiring problem with modifying the alternator connection to the vehicles auxiliary battery. The line has an isolator, diodes, and fuses between the alt and the batt. If I were to divert the line before the auxiliary batt, to the main bank for the cinema, would I need to switch both the positive and negative lines, or just the positives and ground somewhere else on the chassis. Double pole switches are pricey, and I haven't yet found a two way one. Any ideas?

Thanks for all your help. I have taken on board your advice about using a larger battery bank, and keeping the average current demand lower. Hopefully (...) all this will make the power supply safer, more convenient and longer lasting. Right now it is a bit of a headache because I am under time pressure, but it is worth it to try and get it right first time.

[altosack]

You have two sections where you state the amp load for 12V, so I'm confused what it really is. From before, it seemed like about 70A @ 24V would be adequate for your loads, so that would be 140A @ 12V. If your inverter is rated for 2.3 kW continuous, this is about 220A @ 12V with some inefficiency.  Don't worry about the peak value; this is really not needed for sizing the cable.  These are high current values, but are OK for 4/0 cable (ridiculous US wire sizing system - let's see, that's 170k circular mils, or around 100mm2). I should think that this size wire is available from a welding supply (or any solar supplier), and you shouldn't need (2) of the 50mm2 wires. There are connectors designed for paralleling, but they are probably harder to find in this size than the bigger wire.

If you can't get 12V solar panels in time, you could always get a MPPT charge controller to down convert from 24V to 12V. In the US, there's one available called the BZ Products MPPT500 which does 500W at any solar or battery voltage of 12/24/48V as long as the battery voltage is <= the solar voltage; sorry, I don't know what's available over there, but I think you're more advanced in smaller MPPT than we are.

You only need to switch the positive line. Keep the negative connected all the time; although EVs sometimes use a floating ground for their high voltage batteries, especially at 12V you should have a common ground. Definitely, your vehicle alternator should bypass the solar charge controller. While your alternator will not fill the flooded batteries all the way (maybe 90%), as long as your solar panels top them off and equalize them from time to time, I think it will be fine (BTW, this is recommended for home solar electric systems with petrol generators, too; they are not efficient at topping off even if properly regulated).

While you may in fact just leave the vehicle battery in parallel with the solar battery most of the time (you could set the charge controller to its sealed battery setting and not be too hard on the solar battery), you will from time to time need to top off and even less often equalize the solar battery, and you should disconnect the two batteries for that, such as with the battery switch you spoke of.

Best of Luck, I think you're getting there !

Dave