Charging LiFePO4 Battery w/ AC to DC charger from bed outlet?

I'm slowly piecing together my electrical system for my bed cap camping setup, trying to keep it as simple and cheap as possible. My plan is to for now to just maintain/recharge a 100Ah LiFePO4 (with integrated BMS) using a simple AC to DC charger from the bed outlet while driving with the 400W anytime mod.

I know the modified sine wave from the cheap Tacoma inverter doesn't play nice with the power supply stations that accept AC charging, but seems like the ones that have the AC to DC charger (like the brick) built into the cable work fine. I have seen people use a AC to DC power supply unit for charging, but would rather something more compact that wouldn't risk overcharging with mine being a battery not a power supply.

I found this Vevor AC to DC lithium battery charger and reached out to them and they said it would be compatible with a modified/square wave AC input.
https://www.vevor.com/battery-charg...c-lifepo4-smart-charger-for-rv-p_010889683485

Seems like a pretty simple and cheap option? Has anyone tried it (or others) or have any input?

(Disclaimer: I am well aware it is inefficient to go from DC to AC back to DC again and that it would be best to get a proper DC to DC charger and isolator and have solar input as well, but I'm trying to keep things very simple right now, and it would be nice to have an easy backup option to charge via shore power down the road)

 

I'd buy it with a return option, I think a 20A DC Lithium charger is going to require more then the 400W the truck can provide but it's close, here's what I came up with
14V*20A=280W Charger's output.
280W/.8 = 350W Charger efficiency, takes approx 350w to output 280W @ 80% efficiency
350W/120V = 2.91A Current requirement from the 120V circuit. (approximately)
So, this charger would draw approximately 2.91A/350W from the outlet. The trucks 400W mod puts out 3.3A/400W. In the industry we try to build in some overhead, 75% is typically a wise amount which is approx what you'll have everything else being good. The wires and connections will consume some watts, should be small. You can't say for certain what the charger's efficiency is, I put in 80%, a good charger will go up as high as 87%, this one is chinese so you'll have to see. If it works I suspect you'll be right at the limit.

 

Very good point, thank you. I forgot to factor in the efficiency when I was doing my math for the current draw from the bed outlet so I was thinking 280W / 2.3A from the bed outlet was enough overhead from the 400w / 3.3A. They also have a 10A charger that is a bit cheaper so I may opt for that option instead, I'm not super worried about charging time right now. I'll have DC DC with solar input down the road.

 

https://www.youtube.com/watch?v=QQK6141y1e0
Looks like from this guy in the video he was able to put out 57V @ 5.1A, so 290.7W output from the adjustable AC to DC converter/power supply. 280W output from my charger will be just under that so I think I'll order both and return one or get a similar one from Amazon for easy returns. I suppose his power supply could be more efficient than my charger but I'll have to see. Thanks for the important point!

 

I haven't checked it on the Tacoma, and should, but almost every single power inverter I've ever bought except one advertised "120V" but put out 110V.
It seems trivial I know but using the calculations above which are pretty solid, 350W/110V = 3.1818A to get to the same place. If the Taco is pumping out 400 Watts at 110V instead of 120V that's close to the 3.6363A max. Just 0.454545 amps shy - less than half an amp. That means any inefficiency in the wires will steal enough power to make it a no go.

I would check with a volt meter what the volts are at the bed. If it is a true 120, then you are good to go. If it is 110, then you have to stick the landing on this.

Either way, shortest runs possible with the fattest copper you can afford. And don't use the cheaper copper coated aluminum either.

 

Another example: My 5A @ 52V ebike charger pulls 350W , the calculation comes out to 290W plus the inefficiency of 60W so it's 83percent efficient. .

 

Another good point! Thanks I will check the actual AC voltage at the bed outlet tonight. I'm not in a huge rush to order since fortunately I didn't leave this one until a few days before a trip haha.

 

I'll save you the trip. It's 110V.

Just so we are all using the same math here:

• V*A = W
• W/V = A
• W/A = V

Here's when and why it changed ==> https://famuse.co/when-did-110v-become-120v/

I checked on my house and I'm getting 123V at the wall.
I couldn't get the probes to fit in the Tacoma plug (kinda strange, needs more investigation) but I was able to use a SHORT extension cable that had a looser receptacle on the end.
On the one hand, you really want to test it AT THE PLUG to get the true Tacoma Output, but also real world, anything you plug in there is going to have a 2' or 3' cord on it so what you see in my pics is what you would be getting. And drop isn't going to be that much - I measured at the wall with the cord too and it was 123v as well.


Truck was 112V. It did bounce and dip 110.5V to 112V as the AC cycled off and on while idling. But I'd imagine running you get a pretty consistent 112V to 113V.
In the US, power companies are expected to be with in 5% of 120 - so between 115V and 125V. The Taco is pulling up just a little short (electrical joke) of being within 5% of 120V.

That means to get you to the total wattage needed, the amps have to come up.


There's a TON of these charts on line so if you don't like this one, you can go find another one but it shows the drop over distance given wire sizes.



It starts at 5A and we are talking about sub 5 amp ranges.

I looked at the specs for your charger and it doesn't mention amps. Not sure if thats good or bad (too low to worry about / poor product info)

I think you will be fine but I think it is going to run HOT having to buck the amps up to make up for the lack of wattage.

 

Thanks for checking the actual AC voltage and those charts. It's cheap enough and easy enough that I just ordered the 10 ($50) and 20A ($60) Vevor version chargers from Amazon and I'll test them out when they and the battery come in. I'm pretty sure I remember someone on here was using a 10A Victron one from the bed outlet to charge a smaller battery with I believe no issues.

 

Wanted to update this before I forget. Tested out the 10A AC DC charger via my AC outlet in the cab (run by stock inverter) with the 400W anytime mod while driving to work and seemed to work fine. 100ah LiFePo4 battery went from 13.2 to 13.4V in about a half hour, and not that it was a concern but the charging load only dropped my truck battery by .02V (while running) according to my ultragauge when toggling it on and off. No blown fuses or tripped inverter shutoff.

The manual for the two chargers shows 2.3A / 115Vac input for the 10A version and 4.6A / 115Vac input for the 20A version. 529w so sounds like that will be too much for the stock 400w inverter but I may try it ~for science~. I can't figure out why it would draw quite that much besides maybe on initial startup because it says 80% or greater efficiency ( 14.6V*20A=292W output /.8 efficiency =365W input: 3.17A @ 115V or 3.32 @ 110 )but it is cheap Chinese so who knows.

I did run both chargers inside my house to test them out and check temps before determining mounting in my bed camping setup. With charger running for an hour basically enclosed in a cardboard box (simulating one of my compartments) the 20A got up to 155F on the hottest surface of the charger (face it was mounted on) and the 10A got up to 125F. Inside cardboard box faces 115F and 105F, and everything was considerably cooler running not enclosed in the box as expected lol.

 

Great info, I think the 10A charger is a wise move, even if the 20A fired up you'd be close to maxing the inverter out, a little buffer is good.

 

Agreed even the 20A tests ok and somehow works I may stick with the 10A. I'm typically driving a good bit on any road trip and not staying in one place for more than a couple days so 10h recharge time isn't horrible.

 

Last update to this in case anyone else comes across it for reference in the future.

20A ACDC charger did in fact work with the bed outlet, I charged it for probably 4 or 5 drives around 30-45 min each with no issues tripping the inverter switch or fuses or anything.

I did check with a clamp meter though and with 400W anytime mod from the 20A charger I was only seeing 16.4A output with truck idling but plugged into my house AC outlet was outputting proper 19.6A. The 10A charger was putting out 9.74A while idling, same as when plugged into my house AC outlet. This makes me assume the inverter was struggling to put out enough power for the charger. The inverter (in cab) makes a buzzing sound with both chargers, but is noticeably more with the 20A one so that was also a little annoying while driving if I didn't have windows down and/or music on. Between all this and the extra heat the 20A generates I've decided to return it and stick with the 10A one which should be enough to keep up with my needs for now.

 

That's interesting that it drove the 20A, I would have thought it'd fault the inverter instead of the system working at a slightly lower output then the charger wants, that's good to know. I'd still fall back of the fact that it's not an ideal electrical situation to have the inverter running at it's maximum for hours at a time. A failure of the inverter would likely happen when you needed it most (murphy's law).

 

Yeah I thought the same, I don't know enough electrical but assumed maybe just because it's just converting rather than putting an actual load like a motor or something on it it won't draw more than it's supplied. Agreed though regardless of why I'd rather not run it at the limit because I do like having it for convenience.