emergency battery charger? generator to transformer to mppt controller

animattanimatt Solar Expert Posts: 295 ✭✭✭
Living offgrid. After a recent hardware failure which I was prepared for. It got me thinking about possible future failures.

I have one of the generic low frequency inverters chargers from China.


I use Cotek inverter for daily use. For high surge loads I will use the LF inverter as this is where it does shine. Although my surge loads are not very important.


So the inverter charger as an inverter is a backup. But I do use it to charge batteries occasionally from a champion 3500w generator.


Got me thinking of a possible solution, if I have a problem with the electronics inside the inverter charger. It is a 120v ac side and 24v battery bank.
It has a big E I transformer. Supposedly rated at 2000w continuous.


Was thinking to use transformer to hook to generator. Was planning to hook 220v to the transformer and get something like 50vac out of transformer. Not sure exactly ratio of wiring of transformer. That would be something to test. Anyway now hook the lower voltage ac to some bridge rectifier wired in parallel( to allow a decent amperage). The output of the bridge rectifier would then be hooked to my tristar 45a mppt charge controller. Maybe throw in a CAP before the input to controller if needed.


I ask this as I have all this material. I just want a game plan if something goes wrong. Probably get the bridge rectifier ready mounted on a nice heat sink. Something that would get me through a tough spot for a few weeks. I live in rural Mexico and replacement parts are not easily had. My next inverter charger will be a quality one like and outback, xantrex or midnite( If they have an offering at said time).


Anything wrong about what I am thinking?


Thanks for thoughts in advance

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: emergency battery charger? generator to transformer to mppt controller

    Let's see if we can boil this down to the essential.

    You're looking at taking the AC output from a generator, stepping the Voltage down with a transformer, rectifying it to DC, then feeding it through a solar charge controller to provide regulated battery charging. Am I right?

    If so there are some issues. One of them being what the actual V in will be to the controller and whether or not it can handle it. This is where you'd think "MPPT" which an take a wide variety of input Voltage and come up with the right output.

    Unfortunately MPPT will probably exacerbate the other problem which is the controller's sensitivity to the quality of the input power. AC tends to be a bit "noisy" even after being rectified, and will be no place near as "clean" as DC from panels. Some charge controllers may react adversely to this "dirty DC".

    Now someone will come along and tell me I'm wrong, but unless that someone is boB I'm not going to believe it. :D

    (NOTE: many have had the idea of feeding 120 VAC rectified into a MidNite Classic 250 but as far as I know no one has risked a $600 controller to find out if it would work. Sure would make things easier off-grid.)
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: emergency battery charger? generator to transformer to mppt controller

    Pure rectifier+transformer chargers tend to have very bad power factor (takes lots of current only at the peak of the sine waves) and a lot of radio hash (interference created by the switching diodes).

    Of course, they have been in use for many decades and work OK for the job.

    Would I do it in the US--Probably not for a large battery bank. The issues are pretty substantial and if something goes wrong (diodes frequently fail shorted)--You can have some big problems. Plus the heat sinks for the diodes are usually at the voltage of the diode body--So you have to insulate the heat sinks so nobody (and no metal) can short across them.

    You could get into resonance problems (basically creating an inductor/capacitor resonate circuit)--Especially if you add an output capacitor on the rectifier and longer cables to the battery bank.

    Will any of the above "issues" cause you grief... Probably not, but it is difficult to predict.

    If you are careful and monitor the current to make sure you don't have any resonance (with or without the filter caps) and add some appropriate fuses, you probably will be OK.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller

    Wind turbines that feed rectified AC to a Capacitor bank, and then into a MPPT controller, are usually at least 3 phase AC, up to 9 phase, so that there is very low ripple for the MPPT to have to deal with.

    With a single phase, I don't think it's going to work as well.

    I'd be looking more at a inverter/charger that you can feed from the AC of the generator, and let a purpose made charger do that work. Building a high amp charger from parts, is not a simple cookbook recipe project.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • animattanimatt Solar Expert Posts: 295 ✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller

    Building this was incase my inverter charger died. Give me time to buy a new one

    THanks for the replies.

    Battery bank is not very big at only 460ah at 24v.

    Made up of 2 strings of 230 ah. In a pinch i could charge each string seperately.
    Also distance from gen to transformer woukd be about 50ft.
    From trans about 3 feet to BR and additional 3ft to cc.

    Did not think about rectifer shorting out
  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller
    This is where you'd think "MPPT" which an take a wide variety of input Voltage and come up with the right output.

    Unfortunately MPPT will probably exacerbate the other problem which is the controller's sensitivity to the quality of the input power

    Even if you cleaned up the DC, there are issues.... like trying to find the maximum power point of a voltage source. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: emergency battery charger? generator to transformer to mppt controller
    vtmaps wrote: »
    Even if you cleaned up the DC, there are issues.... like trying to find the maximum power point of a voltage source. --vtMaps

    You don't have to let the controller search: most of the good ones can be "fixed in place".
  • animattanimatt Solar Expert Posts: 295 ✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller
    You don't have to let the controller search: most of the good ones can be "fixed in place".

    Was Thinking of limiting the tristar mppt 45a at around 35amps.
    I figure that would leave some room on the cc to not work it too hard, and also other components would not be too taxed. I know it is not loading generator up alot only 5 amp at 220v. But even with poor pf should not be an issue.
    Currently all of this is brain storming and planning just in case.

    Hopefully i will never need to do it, and will find a good deal on an inverter charger before anything happens.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: emergency battery charger? generator to transformer to mppt controller
    animatt wrote: »
    Was Thinking of limiting the tristar mppt 45a at around 35amps.
    I figure that would leave some room on the cc to not work it too hard, and also other components would not be too taxed. I know it is not loading generator up alot only 5 amp at 220v. But even with poor pf should not be an issue.
    Currently all of this is brain storming and planning just in case.

    Hopefully i will never need to do it, and will find a good deal on an inverter charger before anything happens.

    Difference of limiting the output current and fixing the input algorithm so that is has a proper I*V ratio for Voltage-based input (instead of current-based input as with PV's).

    Come to that, you could just build an unregulated DC charge source and keep an eye on it through Bulk. You won't get automatic function or a full 3-stage charger but it would keep the batteries from going dead until something better could be obtained.
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller

    I have done this exact thing, using a 55-ish V 50 Amp 60 Hz transformer, FWB Rectifier, about 200,000 uf capacitor bank, ammeter, circuit breakers on input of transformer, and on the DC output -- fed into the venerable MX-60 MPPT CC.

    MidNite boB cautions to be cautious.

    He has said that this relatively stiff source might cause the CC troubles finding what it thinks is the peak power, and the normal things about having a largish source of current on the CC input. Also, if the capacitor bank on the output should short, it is expected that it will blow up any MPPT CC, etc. This was used on one of the main 1280 AH 48 V banks

    This particular system has just barely enough output V to run the MX-60 to EQ voltage, if the bank is very cold.

    Have used this for the past 8 years, or so, with a Honds EU3000is as the generator, and works great. One can Park the MX at a certain V, but have never needed to do this.

    This was used to recharge the bank after a forced discharge to 50-ish percent SOC of the bank. It did work quite well. YES, it should have a poor Power Factor, but this did not seem to phase (intended) the little EU3000, at 3100 feet Msl.

    A few years ago, on the MN Forum a MidNite Tech Guy posted that he was playing with a generator (or perhaps Grid AC) with a rectifier on the output, with capacitor bank directly into the MN Classic 250 (without the pesky 60 Hz transformer), IIRC. This post was pulled after a day or two ... might have been risky in a couple of ways! My recollection may not be exact, but it was similar to this description. That was a neat project. EDIT: YMMV, Be careful, and realize that stuff does happen, perhaps including breaking some hardware while experimenting, etc Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • animattanimatt Solar Expert Posts: 295 ✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller

    Thanks for the reply vic. Just to confirm the capacitor bank was something like .2F at 60v.

    Cariboocoot what would be an unregulated charger. 120v to a transformer to fwb rectifiers directly to batteries?
    Basically removing the charge controller and adding some resistors in there some where to limit the current. Would i have to be more careful with transformer and get voltage closer to battery bank, or would battery bank basically clamp and hold voltage. If battery bank clamped voltage would there be a need for transformer? I would assume resistor values would change. Thanks
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: emergency battery charger? generator to transformer to mppt controller

    animatt,

    We use a second charger that only puts out about 18 VDC. This one has NO CC. It is a Variable Autotransformer (Variac(r)), to a 120 - 15 VAC transformer, FWB rectifier, capacitors, to a circuit breaker and then an ammeter. This one has automotive jumper cables, allowing attachment to any lead-acid battery.

    In this case, a human is the voltage/current regulator. This works quite well for charging/EQing any battery, and can charge & EQ single cells, if one has access to the individual cell terminals. If the charge controlling human does not use excessive currents vs the Capacity of the battery being charged, the rate of change of battery terminal voltage is fairly slow, and therefore so is the rate of change of charging current, which means that one need not constantly watch the charge current.

    This is also a very useful tool, is easy to build from fairly commonly available parts. The only caution is the obvious need to observe polarity of the jumper cables (as is always the case), and that the output capacitor needs to be charged by the target battery, or initially by the operator, and that a poor connection at the battery terminals could lead to an EXPLOSION, and so on.

    This is not something that most folks here would need, and there are risks associated with the flying leads that need to be correctly polarized when connecting jumpers to the battery terminals. Explosion risks from jumper cable arcs (etc) ... Also there are risks of Thermal Runaway, and no automatic temperature compensation as the charging battery heats and so on. So one needs to be very careful in these situations. There is a large range of improvements that could be made to such a charging system to make it safer and more flexible ... just have not taken the time to do so.

    Good Luck, be careful. Moderators may feel this too risky for a general discussion forum. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: emergency battery charger? generator to transformer to mppt controller

    Vic,

    It is OK... As long as the risks are clearly called out--Then the discussion is fine.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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