$10 DC-DC charger for charging LiFePo4 via 7pin trailer cable?

[sunrise]

I need a charger that can regulate input current to be under 20A. that's the fuse size in my TV.

I just found this cheap boost/step up converter:

3.06US $ 50% OFF|Dc Dc Boost Converter Constant Module Current Mobile Power Supply 250w 10a Led Driver Module Non-isolated Step Up Module - Integrated Circuits - AliExpress

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I also plan to add a relay so I the charging can be turned off when the car is not running:

https://www.amazon.com/Ehdis-Relay-40amp-Spst-Model/dp/B01J53IH5M

it will be triggered by the running light wire (12v)

I should be able to get a 15-20A dc dc charger with adjustable output voltage (plan to set it to 14v) and input regulation (plan to set to 16a)

Is this plan flawed?

Compare to Victron smart dcdc charger, I will lose 2 things with this
1. bluetooth and mobile app. while I love the app for solar charger, this app is useless for dcdc charger, when it's doing its work, I will be sitting in TV driving. plus I can always use BMS bluetooth to watch charging current (need to consider loads of course)
2. charging profile - switch from bulk to float (14.6 to 13.6). - right now I am not even close to worrying about this, I am fighting fuses being blown by charger draw > 20a current. Even this was working perfectly for charging while driving (3-hour or so), I am not worried the converter would damage the battery as I can adjust the charging voltage to below 14.6, for example, 14v.

If this works, then I'd have a super cheap dc/dc charger that can cruise under the red line of TV 20A fuse, right now I am only getting 5-6A from the Victron.

I have so after spent quite sometime with Victron 12/12-18 DC-DC smart charger, and unsatisfactory results(I need to manually set charging voltage based on battery SOC, or I end up with blown fuse), I have decided to look for alternative solutions.
The problem was that the 18A rating really means 'the charger will be able to provide at least 18A current when conditions are met' and this doesn't mean the charger will regulate the input current to 18A. In fact the charger frequently pull 25A and blown fuse several times during my tests.

Thanks!

 

[ultrakiev]

On the link is only 8A. As far as I know this cheap converter, not current limiting, it will fry instantly on high battery demand. You can get Renogy 20A DC-DC charger it has the function of reduced output down to 10A just need to provide power to LC terminal.

 

[sunrise]

Sorry I posted wrong converter, the 20a one is this

13.84US $ 25% OFF|1200w 20a Dc Converter Boost Step-up Power Supply Module 8-60v To 12-83v With Heat Sink - Inverters & Converters - AliExpress

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a.aliexpress.com

 

[sunrise]

Then one on the link is only 8A. As far as I know this cheap converter, not current limiting, it will fry instantly on high battery demand. You can get Renogy 20A DC-DC charger it has the function of reduced output down to 10A just need to provide power to LC terminal.

I have researched this one as well, it only limit output current and to a fixed amp value.

13.84US $ 25% OFF|1200w 20a Dc Converter Boost Step-up Power Supply Module 8-60v To 12-83v With Heat Sink - Inverters & Converters - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

 

[ultrakiev]

I have researched this one as well, it only limit output current and to a fixed amp value.

13.84US $ 25% OFF|1200w 20a Dc Converter Boost Step-up Power Supply Module 8-60v To 12-83v With Heat Sink - Inverters & Converters - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

It says it's current limiting but I would test it before permanently mounting it , usually, current limiting power converters are premium product features. Hard to believe, but it's probably worth trying. Just need to make some housing for this.

 

[ultrakiev]

You can combine it with this module - https://www.aliexpress.com/item/33052566618.html?spm=a2g0s.9042311.0.0.359c4c4dfzEEdC
to get proper cycling for your battery. I have a couple of chargers equipped with them that works really well for lead-acid batteries desulfation and lifepo4 charging you can set the voltage to 14.6 to cut out completely.

 

[sunrise]

You can combine it with this module - https://www.aliexpress.com/item/33052566618.html?spm=a2g0s.9042311.0.0.359c4c4dfzEEdC
to get proper cycling for your battery. I have a couple of chargers equipped with them that works really well for lead-acid batteries desulfation and lifepo4 charging you can set the voltage to 14.6 to cut out completely.

thanks, I see it’s got 3 buttons for settings, does this module save settings to some storage? Or I have to set it every time?

 

[snoobler]

This might work too:

Amazon.com

13.8V constant voltage output, wide input (8-40V), 10A limited output.

Single cells can get to 99% SoC @ 3.45V (13.8V for 4S) with a long slow charge.

Going to lithium, converter question....

The majority of LFP charging occurs below 13.6V. PD has a motivation to sell new converters. I have a motivation to enable informed choices. I have personally charged a single cell to 95% capacity @ 3.4V. It started at 0.33C...

diysolarforum.com

 

[sunrise]

This might work too:

Amazon.com

13.8V constant voltage output, wide input (8-40V), 10A limited output.

Single cells can get to 99% SoC @ 3.45V (13.8V for 4S) with a long slow charge.

Going to lithium, converter question....

The majority of LFP charging occurs below 13.6V. PD has a motivation to sell new converters. I have a motivation to enable informed choices. I have personally charged a single cell to 95% capacity @ 3.4V. It started at 0.33C...

diysolarforum.com

Let me clarify my goal a bit more

The absolutely minimal goal: when my coach battery is really really low, and if I travel in cloudy/raining days(common in PNW) or during nights, I need the charger to provide at least 5A charging current, to cover the fridge (3A, when running) and with a bit extra to bring the battery a little more charge so if breakaway happens(unlikely but possible) the battery has enough power for a few seconds to stop the trailer.

If I manually limiting the current by setting the Victrion DCDC charger bulk voltage to something like 12.8-13v, I probably will achieve the minimal goal, but that sucks, it's a fairly expensive device @$150. charging @ 5a with still not very reliable current limiting (my battery could be very low SOC and even 13v might blow TV fuse)

The acceptable goal: input is limited reliably below 20a, and it charges battery @ > 10A.

The stretch goal: Input is limited reliably below 20a, and I get charging current ~12-15A.

I don't really plan to use the boost converter's step up feature, my alternator puts out 14v and I am perfectly happen with that, so I will just set the max input current to 18a, and output voltage to 14v(same as input), or even 13.8v. I hope with this the converter is operating at its ideal efficiency condition. and would draw similar amps as output current.

 

[rickst29]

Sunrise - if your Trailer already has a Solar withj an MPPT, you could go a completely different route by using the scheme which I installed into my 4Runner and TT several years ago (it's been working great). My "Dashboard Switch" raises the Voltage of the Trailer Battery Charge (TBC) through a boost regulator under the hood, from 4Runner's ECM Voltage (13.4V to 14.2V, depending on the state of the 4Runner engine-start battery).

Within the Trailer, the TBC doesn't go straight to the batteries. It goes into a "detector relay", which has a 24v coil which can handle 36v input for indefinite hold time. The "power" leads of that relay just carry 12v from my own trailer LFP battery bank, and become the coil-switching leads into a pair of 5-pin automotive relays. The first Relay switches the "TBC" from the battery (and the rest of the Trailer) to become in input lead for the second Reley; The scond Relay switches my SCC "PV +" from the real panels (490 rated watts at 65 Volts MPPT) to use the "36 Volt" TBC power source instead. My SCC adjusts almost instantly, in finding the 36V optimal voltage, and runs "fake Solar" from the Toyota for as long as I have the dashboard switched engaged. (Or until I turn off the Toyota, or disconnect the Bargman).

The Relays only cost $30, and a "12v" -> 36.0V regulated Converter with 15A maximum power capability costs around $50. The Trailer switches automatically, between 12-volt and > 24 Volt TBC power sources (you can also tow your Trailer with "normal" non-switching Vehicles, and your TV can tow other trailers -- just don't hit the dashboard switch. My max draw through the "540 Watt converter" is limited by the MPPT maximum current of 30A *14.6V output, roughly 450 watts of draw on the Converter (which stays cool and efficient at that load. For only about $80 of parts, my scheme is mostly a question of time spent under the hood -- and in the dashboard -- of the Tow Vehicle. But the big effort was installing the dashboard switch, and you're ALREADY planning to do that. Could this scheme meet your needs better, but battery charge under the control of the MPPT SCC?

 

[ultrakiev]

thanks, I see it’s got 3 buttons for settings, does this module save settings to some storage? Or I have to set it every time?

It does keep settings.

 

[rickst29]

Within the TV, of course, the dashboard switch controls two switching power relays. The first one, from the battery (on a BIG wire, with a big fuse in it) chooses to go into the Boost Converter-Regulator, or "directly" to the second Relay. The second Relay (switched at the same time) chooses to take power from the "direct" wire, or from the Boost Converter output. Your existing 20A fuse is completely bypassed, You use a new 20A fuse on the second Relay "common" output, where it connects to the now disconnected "TBC" wire.

 

[sunrise]

Sunrise - if your Trailer already has a Solar withj an MPPT, you could go a completely different route by using the scheme which I installed into my 4Runner and TT several years ago (it's been working great). My "Dashboard Switch" raises the Voltage of the Trailer Battery Charge (TBC) through a boost regulator under the hood, from 4Runner's ECM Voltage (13.4V to 14.2V, depending on the state of the 4Runner engine-start battery).

Within the Trailer, the TBC doesn't go straight to the batteries. It goes into a "detector relay", which has a 24v coil which can handle 36v input for indefinite hold time. The "power" leads of that relay just carry 12v from my own trailer LFP battery bank, and become the coil-switching leads into a pair of 5-pin automotive relays. The first Relay switches the "TBC" from the battery (and the rest of the Trailer) to become in input lead for the second Reley; The scond Relay switches my SCC "PV +" from the real panels (490 rated watts at 65 Volts MPPT) to use the "36 Volt" TBC power source instead. My SCC adjusts almost instantly, in finding the 36V optimal voltage, and runs "fake Solar" from the Toyota for as long as I have the dashboard switched engaged. (Or until I turn off the Toyota, or disconnect the Bargman).

The Relays only cost $30, and a "12v" -> 36.0V regulated Converter with 15A maximum power capability costs around $50. The Trailer switches automatically, between 12-volt and > 24 Volt TBC power sources (you can also tow your Trailer with "normal" non-switching Vehicles, and your TV can tow other trailers -- just don't hit the dashboard switch. My max draw through the "540 Watt converter" is limited by the MPPT maximum current of 30A *14.6V output, roughly 450 watts of draw on the Converter (which stays cool and efficient at that load. For only about $80 of parts, my scheme is mostly a question of time spent under the hood -- and in the dashboard -- of the Tow Vehicle. But the big effort was installing the dashboard switch, and you're ALREADY planning to do that. Could this scheme meet your needs better, but battery charge under the control of the MPPT SCC?

Thanks, i do have an MPPT scc, this will take me some time to grasp, but fresh ideas are awesome!

 

[hour]

Would the length of wire and gauge between tow vehicle and trailer not result in current limiting?

 

[chrisski]

Sunrise - if your Trailer already has a Solar withj an MPPT, you could go a completely different route by using the scheme which I installed into my 4Runner and TT several years ago (it's been working great). My "Dashboard Switch" raises the Voltage of the Trailer Battery Charge (TBC) through a boost regulator under the hood, from 4Runner's ECM Voltage (13.4V to 14.2V, depending on the state of the 4Runner engine-start battery).

I would be very interested to find out production results. Also, this seems like this is not through the 7 pin, but wired separately, which would allow bigger wire to cut down on voltage loss. I tried to follow the wording, but a block diagram would be better.

Would the length of wire and gauge between tow vehicle and trailer not result in current limiting?

I looked at charging through my seven pin, and after running through a wire size calculator, I came up with the conclusion that no real charging happens there, and that would be nothing more than a float charge at best. With 458 AH of battery charging, the numbers I came up going through the seven pin would not make a dent in any loss. My RVs built in converter can push 15 amps if I'm in need of charging and out comes the generator for that.

 

[rickst29]

Sunrise, let me give you a couple of diagrams to help make it more clear. Here is the Tow-Vehicle diagram, from a few years ago when I boosted to only 24 Volts. I now boost to 36 volts, at lower current.

 

[rickst29]

Hour, and Chrisski: In a Trailer connected to the TV, this DOES use the regular Bargman cable, but only at a load of about 12-13A (when the target MPPT is limited to 28-30A maximum output current, charging 14.4-14.6V LFP batteries). Because the current is low, compared to the TBC wire size within the Bargman, voltage drop is minimal. From blown Bargman cords I've fixed in the past, I've usually seen 14-AWG, overloaded by pulling 30A currents and burning at the plastic connectors. Some might be bigger, and maybe some "sucky" small cables have even smaller TBC, but that's what I've seen in just a few cable repairs which I've done.

 

[rickst29]

Here's the relatively 'complex' version of the Trailer, which ALSO uses a 'primary' relay to accept 12V (from TBC) directly into the 12V system bus. I've now deleted that one, I use only the 'detector' and the 'Solar Switcher' relays in my own Trailer. (It's never towed by anybody else, always gets towed in 36V mode by me, and would still have "500W" of solar panels connected if I forgot to flick the switch.) Since those days, I've upgraded to a 30A MPPT as well.

 

[rickst29]

Here's an ancient picture with my old MPPT, after cutting back to just two Relays. (The ancient Rogue was a "big-iron" design with a huge inductor, and couldn't compete with more modern, cheaper-to-build designs. It was indestructible and absolutely trustworthy, but it weighed a ton.)

 

[ultrakiev]

thanks, I see it’s got 3 buttons for settings, does this module save settings to some storag? Or I have to set it every time?

For 6$ I turned old transformer charger to a smart programmable almost all types batteries charger.