DIY back up charger

**SunEagle** · 10-16-2021, 11:22 AM

You would need to know what the maximum voltage (AC or DC) that your inverter allows. Since most grid tie inverters are looking for DC I don't know of any simple transformer that would work without an AC to DC converter between it and the inverter.

Most people just have an AC connection for their generator through a transfer switch that will disconnect the home from the grid while the generator is running. That requires a lot less equipment which increases the efficiency of the generation system.

**Takis** · 10-16-2021, 01:15 PM

No, it's not grid tie. All of the system is off grid.
I have separate charge controller and separate inverter but none has charging by mains/generator.

I'm just thinking of a 230v ac transformer and rectify bridge to 120v DC. I will not build it myself. I will order it from a guy who builds transformers.

My main concern is that if the pv input of the charge controller can accept just dc and charge the batteries passing all charge states.

**SunEagle** · 10-16-2021, 04:28 PM

Originally posted by Takis

No, it's not grid tie. All of the system is off grid.
I have separate charge controller and separate inverter but none has charging by mains/generator.

I'm just thinking of a 230v ac transformer and rectify bridge to 120v DC. I will not build it myself. I will order it from a guy who builds transformers.

My main concern is that if the pv input of the charge controller can accept just dc and charge the batteries passing all charge states.

Well most CC's do not work if you just put a DC voltage power supply to it's input. They were really designed for a solar panel as input.

IMO just using a quality battery charger powered by the generator will get you better results in keeping your battery working.

**sdold** · 10-17-2021, 12:20 AM

Originally posted by SunEagle

IMO just using a quality battery charger powered by the generator will get you better results in keeping your battery working.

I agree with SunEagle, this is the way many off-grid facilities work, there is a generator that produces AC power, and that powers a charger for the batteries. Not a transformer and rectifiers that someone built for you, but a real AC-powered battery charger built by a reputable company like Newmar, Eltek, Sageon etc.

**Mike90250** · 10-17-2021, 12:53 AM

> I'm just thinking of a 230v ac transformer and rectify bridge to 120v DC. I will not build it myself. I will order it from a guy who builds transformers.

Nope. Most likely won't work. The MPPT in the solar charge controller will try to change the voltage and remeasure the current every couple of minutes. So the weaker thing will break, either the generator/AC-DC or the charge controller being used improperly.

You could hook your AC-DC power supply right to the battery and charge it, but I suggest getting a factory built AC-DC battery charger, not homebrew

**Takis** · 10-20-2021, 01:23 PM

Originally posted by Mike90250

Nope. Most likely won't work. The MPPT in the solar charge controller will try to change the voltage and remeasure the current every couple of minutes. So the weaker thing will break, either the generator/AC-DC or the charge controller being used improperly.

What if i use a spare PWM charge controller after the rectify bridge ?
Will that have the same problem as Mppt ?

**sdold** · 10-20-2021, 04:00 PM

Originally posted by Takis

What if i use a spare PWM charge controller after the rectify bridge?

It depends. What do you mean when you say "rectify bridge"? A bridge rectifier is part of a power supply
or charger, and will be followed by various filters, voltage regulators and current limiters. It's not something
you will be connecting directly to.

So three questions:
1. What do you mean by "rectify bridge"?
2. Assuming it's a power supply or charger, what is its current limit value and voltage?
3. What is the make/model of the PWM controller and what is its maximum current capability?

**Mike90250** · 10-20-2021, 07:05 PM

By the time you trial and error several DIY systems you will have learned that unschooled electronics is costly,
If you have a 20A power supply and a 15A PWM controller, the controllers fuse will pop ( or something else) . Note i am speaking about a simple Power Supply, which is more than a transformer and bridge diode.

If you have a 20A PS and a 25A PWM controller, the controller will overload the PS and again, something expensive fries.

Iota makes a regulated supply rated for 100% Load, regulated battery charging.

You need a supply rated for Voltage & Current limit, so that it doesn't blow up after charging batteries for 20 minutes. Most supplies are rated for an "average" load, not a full load.
Even generators have a Surge load for 5 min, Max load 20 min and Normal load for infinite.

**sdold** · 10-20-2021, 08:20 PM

We learned the hard way not to exceed the rated current on 12A PWM controllers, by adding a 2nd 100W parallel panel and blowing the FETs.

**Takis** · 10-21-2021, 02:49 AM

Originally posted by sdold

1. What do you mean by "rectify bridge"?

I mean bridge rectifier as you mention.

Finally I calculated the cost of transformer+bridge+pwm and is 30/40 dollars less than a ready medium quality battery charger.

So the risk of throwing money is not worth it.

Thank you all.

DIY back up charger

DIY back up charger

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