Help me design the lowest cost way to dump current into a battery bank - Page 1
Forum section: electronics / projects

author: Lightages
author_rank: Unknown
timestamp: January 13, 2012, 08:40:05 pm
content: Here is the situation. I live in the Atacama Desert in Chile and run my house strictly from solar power. I have 16x 210W solar panels feeding 3x4(48V) 12V 250AH AGM batteries through a Xantrex MPPT60-150 solar charger. The batteries feed a Xantrex XW4548 220V inverter/charger. When it is clear here (95% of the time) I have surplus energy and no problems getting the batteries back up to charge everyday. When it is cloudy here during the cloudy period every year it can be so for days.During this cloudy time the panels can't keep up and I need to supplement the charge with a generator. The way this normally works with the Xantrex XW4548 is it will switch over to bypass allowing the generator to supply the house directly while at the same time taking power from the generator and charging the batteries. The problem with this scenario is that the generators available here all tend to be rather poor on the AC quality and the inverter doesn't necessarily like it.So I have been mulling over a way to take the generator output and dumping current into the batteries the cheapest way possible. All I would like the generator to do is get the battery back up to end of bulk phase voltage, and then get shut down. This means that no special regulation is needed for voltage or DC smoothing.So the simplest thing I could think of was just rectifying the AC output of the generator and passing the output through the simplest current regulator. The generator only has a specific amount of output it can supply so a current regulator is needed to prevent the 0.01 ohm battery load from just loading the generator down and stalling it or kicking the circuit breaker.I am not sure of what will actually happen when a rectified 220VAC voltage is supplied through a current regulator to a battery bank that is nominally 48V and maximum allowed 59V. The current regulator should just limit the current and the voltage should just follow the charge state of the batteries. Once the batteries reach their bulk charge end voltage of 58.8 volts, one of the Xantrex devices will shut down the generator.This rectifier/current regulator will be taking a 220VAC input and should be able to supply an adjustable output current from say 10 amps to up to 50 amps.Oh yes, I don't want to be throwing away any power through waste heat or otherwise in the circuit, or as little as possible.Any ideas?

author: Unknown
author_rank: Unknown
timestamp: January 13, 2012, 08:50:09 pm
content: Quote from: Lightages on January 13, 2012, 08:40:05 pmA DC generator, like in one of those engine powered welding machines....

author: Lightages
author_rank: Unknown
timestamp: January 13, 2012, 08:53:12 pm
content: I appreciate the response, but I am trying to come up with a minimal circuit, a safe one, to do this. I already have a generator and don't feel like buying a new one just to charge the batteries when it is necessary to supplement the solar charge.

author: rr100
author_rank: Frequent Contributor
timestamp: January 13, 2012, 08:53:26 pm
content: You can't limit the input current to something larger (I assume you don't get 220V 50A at the input but you would like up to 50A at 48V).As I also think cost is part of the equation so probably a simple transformer + rectifier would do (you do get AC as input, no matter how poor, isn't it?). This is the way AC voltage is converted almost all the time around us, speaking about high voltages and power.Or you can experiment with a variac; not that efficient or robust but might help you to find the sweet spot. And they aren't that expensive.

author: Lightages
author_rank: Unknown
timestamp: January 13, 2012, 08:59:19 pm
content: Quote from: rr100 on January 13, 2012, 08:53:26 pmDOH! Of course you are correct. It would need a transformer to get higher current. I would need a 2kVA transformer or bigger too. But after that and the rectifier, this simplest current regulator is still what I am looking for.

author: mobbarley
author_rank: Regular Contributor
timestamp: January 14, 2012, 12:16:13 am
content: 48v is a common battery bank config for telco equipment - there are probably a lot of large 48v supplies or chargers on ebay.

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 01:44:55 am
content: Well after doing some searching for commercial chargers I have run across a couple that are cheaper to buy than it would be to build. One is a Mastech Model GCA48V30A.A transformer by itself with nothing additional will probably cost the same.Still open to ideas though

author: Psi
author_rank: Super Contributor
timestamp: January 14, 2012, 02:37:43 am
content: Time for some out of the box thinking....Could you do some sort of switched system where a bank of capacitors are put in series across the rectified DC from the AC generator.They charge up and are disconnected, then put in parallel and connected across the battery bank.Then the process happens again.Obviously relays are out of the question, so some sort of semiconductor switching system would be needed.But all you would need is a bank of caps, some switching system and a 555 timer

author: IanB
author_rank: Super Contributor
timestamp: January 14, 2012, 04:57:30 am
content: Quote from: Lightages on January 13, 2012, 08:40:05 pmDo you know what is inside the Xantrex power input stage and what it is about the generator output it doesn't like? Is it noise, or dips, or spikes, or...?I ask because these days most devices use switch mode power supplies and the first thing they do with the input supply is rectify it and smooth it. If the Xantrex has a switch mode input you could probably feed it DC as just as well as AC. So you could take the generator output, rectify it and smooth it with a filter capacitor, and feed this to the Xantrex. That would give a fairly clean supply free of dips or spikes and maybe it would work better.This is in the spirit of thinking outside the box, and of course it might not work at all.

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 05:01:14 am
content: Hmmm, a slow switching power supply.....The biggest problem I can see with that is the generator would go nuts trying to deal with the large dynamic load change. Maybe it could work with two banks of caps working alternately. It would need some big rectifiers, two big capacitor banks, solid state relays or triacs or scrs, some kind of controller....I am sure this would add up to more cost that a couple of hundred dollars.

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 05:05:41 am
content: Quote from: IanB on January 14, 2012, 04:57:30 amThe Xantex inverter does not just use the generator for charging, it also passes the power on to the AC loads directly. It only does this though if the quality of the power is good enough. The output of generators like the one I have are pretty dirty. The have problems with high distortion, phase wobble, and other nasties.

author: kaz911
author_rank: Super Contributor
timestamp: January 14, 2012, 05:20:47 am
content: how about just adding one of these ?http://www.voltage-converter-transformers.com/voltage-stabilizer.html

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 05:39:03 am
content: These we would be ferroresonant power supplies. They work by having a transformer that is working in saturation and the amount of saturation is controlled by another winding. They do control voltage but at the expenses of AC waveform purity, and that is putting it lightly. There is no way the inverter would lock on to one of these.Edit:Actually it is possible that these are the more modern design with better AC waveform output.

author: kaz911
author_rank: Super Contributor
timestamp: January 14, 2012, 05:52:14 am
content: They do have a deluxe version as well - but only to 3000w - but they are heavy as...I do not think you are the first with that kind of problem. Have you tried asking Xantrex for a possible solution?My company has two offices in Dhaka where we often need to use generators - since power delivery is very unstable. And we have voltage stabilizers on them - but not feed into batteries/inverters but into computers, severs and printers. :-) So far no blow ups in 6 years.The advantage of a good stabilizer on your system is you can still use the generator auto-start in the Xantex.

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 06:00:41 am
content: Quote from: kaz911 on January 14, 2012, 05:52:14 amXantrex has relaxed the power quality of the inverter this last year but it is still not enough sometimes. I also forgot to mention that I use a UPS on a power sensitive network attached RAID5 system. The UPS will not run on the generator, it insists on using the batteries.Quote from: kaz911 on January 14, 2012, 05:52:14 amI might try one of these. They are cheap enough but I would really prefer to charge the batteries directly.

author: kaz911
author_rank: Super Contributor
timestamp: January 14, 2012, 06:06:46 am
content: :-) well the SIMRAN does not get the best "reviews" but I found quite a lot of them online.I know ours in Dhaka break down very often - but they are used to it - so quite a few guys know how to fix it. But I do not know which brand it is.Question - on the normal days - how much surplus power do you have? Normal solar design says calculate battery capacity for 5 cloudy days. I know that is expensive and heavy.

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 06:27:59 am
content: The normal way of calculating for solar systems is:1. Determine power usage2. Assume no more than 50% discharge of batteries and 2 - 5 days of no charge use, battery size should equal 4 times consumption or so3. Use enough panels to supply no less than 5% the C/20 of the batteries.My consumption is variable. I can be more conservative when it is necessary so I can get 4 days out of my batteries if I need to without problems. Even then, the less you discharge the batteries the longer their life. Discharge any normal lead acid battery below 50% and you are looking at a very short service life. AGM type batteries will take this a bit more but their life is also shortened.My panels can generate around 3000W during peaks hours and an over all 22kwh per day easily, if not more. So I can use quite a bit of energy every day and get the batteries topped up too.It is when we have our week long cloud cover periods, sometimes two or three periods during the bad season, no amount of panels will keep up. We are approaching this time and just had a taste of it last week. This is why I am trying to work this out now.

author: kaz911
author_rank: Super Contributor
timestamp: January 14, 2012, 07:01:41 am
content: ahh okay - not like where I am.. I have sun for about 97% of the days - but clouds in patches of a day or so.But due to shade and very high heat and not to much space etc. I had to go to 190Watt mono panels to drive a 12v/1.5Amp constant load - but I have skimped on battery (105ah AGM) since even on cloudy days I get about 50-60% of normal charge. So once in a while I drop down to 40% battery capacity - but maybe once a month so far. But I do feel "annoyed" when my charger starts CV charging - ;-) all that good excess current going nowhere.....

author: SeanB
author_rank: Super Contributor
timestamp: January 14, 2012, 10:19:42 am
content: Using a transformer and bridge to get a voltage a little above your full battery charge voltage is easy. Current limiting is easy enough if you get a choke to do the current limiting. If you have 2 identical transformers ( and i see somebody has 208V ups units on the forum that probably would be a good source of the transformers) and use the one as a choke, either on the primary or the secondary. An old adjustable welder will give you a variable inductor to use as a current limit as well. Just be aware the primary will have to be disconnected and isolated as it will have a high voltage on it. You will need high power diodes, most likely each on a big heatsink, preferably with a fan on each to do forced air cooling. They will need to be isolated from each other of course. Other method is a big wirewound resistor, or a lot of steel wire wound on a heat resistant former. I used to do generator testing, where the test load was an iron bed frame, with a short section of bakelite across a cut to provide the insulation, and a whole lot of steel springs ( about 1/4 inch diameter and 4 foot long) that was the load. That would glow red on full power, 28V at 3kA being full power test. The 115V resistor was a steel drum filled with salt water, with 3 steel rods on the lid to contact it. Run to boiling and adjust the rod immersion to adjust current.

author: Lightages
author_rank: Unknown
timestamp: January 14, 2012, 06:37:42 pm
content: @SeanBInteresting ideas on current limiting. Yes, a transformer and diodes is a no brainer but still not cheap as I want. The current limiting needs to be easy to manage, set and forget. Thanks for the ideas.@kaz91197% clear? Where are you?

author: kaz911
author_rank: Super Contributor
timestamp: January 14, 2012, 07:54:24 pm
content: I am in Abu Dhabi - United Arab Emirates :-) So winter daytime 20-25 - summer daytime 40-50 deg c. and about 360 days of sun pr. year (with some scattered cloudy days.Biggest problem is heat - so total panel loss in a typical day can be up to 30% due to heatloss and charge efficiency. I use a Sunsaver MPPT with 2x80watt + 1 x 30 watt - in parallel / that gave better efficiency pct than 2 x 80 watt in series. (Cant add the 30watt to a series setup) but charge loss was much higher.Everything is logged with Morningstar's MSView - via RS232/Modbus interface.On my little boat I have a Sunsaver Duo - with a 28watt roll-able panel on it. Before that I used to replace batteries every year due to heat + battery drain. Now they seem to stay in shape.Then I have an experimental 50 watt panel mono crystalline panel (made in china) with a 55ah battery and an old Steca 6amp charger on (salvaged from a wireless project) - I use that for micro controller fun./k