DC:DC controller - which one?

CharlieRN · June 2021

As part of my switch to new LiFePO4 battery, I plan to add a DC:DC charger and am looking for feedback on the various offerings out there. I don't see a need for anything other than a 20 amp unit.

The Renogy gets generally good reviews and is the least costly alternative. The reviews indicate that it performs quite well with the only negatives being the lack of an auto-detect feature (necessitating an additional wire to the ignition side of the TV), and no temperature sensing, which I'm told is not really a concern in the case of Li batteries.

On the other end of the price spectrum are the Redarc and Sterling units. The both appear to be very robust units that work well but are nearly four times the price of the Renogy.

In the middle price-wise is the Victron Orion Tri-smart. It's about 50% more expensive than the Renogy, but the extra money gets your auto detect and BT capability.

Would be great to get more information and feedback from the T@B community on this subject.

DenverJaguar · June 2021

Is something like this really necessary if you have solar? Wouldn't solar be topping off the batteries at all times?

CharlieRN · June 2021

That's actually a good point @DenverJaguar. The short answer is "no" it's not actually needed, but in the sense that it's not absolutely necessary to have the ability to charge up your camper battery from your TV. Both your solar array and the WFCO can adequately charge the battery if you have a decent solar system and sufficient sunlight, or in the case of the WFCO, a shore power connection. The DC charge controller is necessary only if you want or need to charge the battery from the TV, and do not want to risk damage to the alternator.

If you don't care about having that ability, you can simply eliminate the charging connection from the TV to the camper in the 7 pole. In fact, this is precisely what I intend to do as I sort through which DC:DC charger to get. I'm still in the crawl phase of trailer camping, and plan to boondock only two or at most three days.So I start any trip with a fully charged the battery from being connected to shore power. When camping, the combination of the 320's solar panel's 105w and my Renogy 100w solar suitcase should be able to keep the battery sufficiently topped up for a 2-3 day trip, even if cloudy.

Once we start doing longer trips, the off-grid days are likely to increase. To have an extra measure of security for poor sunlight conditions, I think I want the ability to charge the camper battery from the TV and hence the need for the DC:DC charger.

crampshaw · June 2021

I use a Ctek d250s dc-dc unit. It’s a 20 amp unit that’s worked great.

Marceline · June 2021

CharlieRN said:

That's actually a good point The DC charge controller is necessary only if you want or need to charge the battery from the TV, and do not want to risk damage to the alternator.
If you don't care about having that ability, you can simply eliminate the charging connection from the TV to the camper in the 7 pole.

Why do you believe that there’s a risk of damage to your alternator?

CharlieRN · June 2021

@Marceline - The short answer is that Battleborn suggests either using a DC:DC isolating charger or manually isolating the LiFePO4 battery from the charging system to prevent damage. I believe that this is only an issue for lithium batteries.

My understanding is the charging demands from Li batteries stresses most modern alternators that are load sensitive, with two possible results. One is that the Li battery doesn't get fully charged, something that is merely inconvenient and unimportant if you have solar that will top things off. The other is that the alternator circuitry that is happy charging at 13.7 - 13.8 volts gets damaged by constant calls for much higher voltage (14.4 - 14.6 v) from the lithium battery.

My understanding also is that this often isn't an issue because of the small gauge charging wire in many 7-pole connectors: the narrow wire doesn't allow the greater flow. My connection is via 10AWG cable though, which is a different situation. I'm far from an expert in all of this and am really just regurgitating what I learned from BB, via the Will Prowse solar pages and other info from those with much more more knowledge about this than I have.

N7SHG_Ham · June 2021

If you have other Victron components, I would probably go that way.

CharlieRN · June 2021

Thanks to all for the comments.

I think I'm going to go with Victron because of its auto-detect feature and because I already have their 75/10 solar controller and the BT app. There is a cost penalty vs the Renogy but I'm willing to pay a bit more for the Victron's easier install and the convenience of remote monitoring.

dhauf · June 2021

I installed the Renogy 20amp DC to DC convertor here: https://www.renogy.com/12v-dc-to-dc-on-board-battery-charger/
It has a limiter you can wire to the positive terminal to set it to 10amps which is what I am currently running and it works perfectly. The quality seems very good and have had no issues. Plus I purchased it for $90 so it was relatively inexpensive.

m_lewis · June 2021

A bit of educational data: While a lithium battery does require a slightly higher voltage to reach full charge, the battery does not "call for" a higher voltage. It does present a much lower internal resistance to the charging components. As a result, a LiFePO battery can absorb much higher currents than an AGM or other lead-acid batteries. It is the higher current that *might* damage your alternator.
The alternator's voltage regulation circuitry controls the voltage, not the battery.

Dutch061 · June 2021

@CharlieRN, Victron offers a couple of variations along with different amperages. Regardless of the size, make sure to order the model with the "isolated ground".

Brad

CharlieRN · June 2021

Thanks again to all for the responses. As my previous post indicated, I think I'm going to stick with Victron and in fact, have the isolated version of the 20 amp Orion Tri-smart in my cart as I ponder things.

A related question about wire size. I know that the 20 amp rating for the charger relates to the units max output - is it safe to assume the input side sees the same 20 amps max? If yes, I'd think I'd be fine with 10 AWG. Is this correct?

dhauf · June 2021

CharlieRN said:

Thanks again to all for the responses. As my previous post indicated, I think I'm going to stick with Victron and in fact, have the isolated version of the 20 amp Orion Tri-smart in my cart as I ponder things.
A related question about wire size. I know that the 20 amp rating for the charger relates to the units max output - is it safe to assume the input side sees the same 20 amps max? If yes, I'd think I'd be fine with 10 AWG. Is this correct?

Victron is a solid choice as well and yes 10AGW is rated for up to 30amps at 12V. You also have to factor in the length of wire run as well, the longer you go the max amperage drops after a certain length and so can voltage. But for the T@bs you probably won't have to worry about that, since most wire runs are relatively short. Input amperage is directly proportional to output amperage. You'll only be able to draw 20amps max on the input side if you are getting 20amps max on the output side. The DC to DC converter just insures that your voltage stays at a constant 14.6V for the best results in charging a lithium battery.

CharlieRN · June 2021

m_lewis said:

A bit of educational data: While a lithium battery does require a slightly higher voltage to reach full charge, the battery does not "call for" a higher voltage. It does present a much lower internal resistance to the charging components. As a result, a LiFePO battery can absorb much higher currents than an AGM or other lead-acid batteries. It is the higher current that *might* damage your alternator.
The alternator's voltage regulation circuitry controls the voltage, not the battery.

Thanks for the insight and the correction.

The charging system on my car seems to be one that is unusual. From what I read, the current and voltage regulation system is internal to the alternator, but the current and voltage output is controlled by the ECU. I frankly have no idea whether that makes it more or less sensitive to damage from use with a LiFePO4 battery, but I do know that the cost for a replacement alternator is roughly the size of my monthly mortgage payment. This is mostly due to the difficulty of installation in an incredibly crowded engine bay but in any event, I'd prefer to spend a few dollars to help keep my [relatively new] replacement alternator out of any potential harm.

dhauf · June 2021

CharlieRN said:

m_lewis said:

A bit of educational data: While a lithium battery does require a slightly higher voltage to reach full charge, the battery does not "call for" a higher voltage. It does present a much lower internal resistance to the charging components. As a result, a LiFePO battery can absorb much higher currents than an AGM or other lead-acid batteries. It is the higher current that *might* damage your alternator.
The alternator's voltage regulation circuitry controls the voltage, not the battery.

Thanks for the insight and the correction.
The charging system on my car seems to be one that is unusual. From what I read, the current and voltage regulation system is internal to the alternator, but the current and voltage output is controlled by the ECU. I frankly have no idea whether that makes it more or less sensitive to damage from use with a LiFePO4 battery, but I do know that the cost for a replacement alternator is roughly the size of my monthly mortgage payment. This is mostly due to the difficulty of installation in an incredibly crowded engine bay but in any event, I'd prefer to spend a few dollars to help keep my [relatively new] replacement alternator out of any potential harm.

No problem, ECM controlled alternators are actually fairly common now in most of the newer cars. The voltage regulator in the alternator talks to the ECM to adjust voltage output, which makes it more efficient. This won't affect anything at all when it comes to the lithium battery charging when you have a DC to DC converter in line. The DC to DC converter has the ability to buck, meaning drop voltage if it's higher, than the set point (in this case 14.6VDC) or boost the voltage if its lower than the set point. So if you car for example puts out 13.6VDC as it's running, the DC to DC converter will boost this voltage up to the specified 14.6VDC value. Before installing mine I bench tested everything using a variable DC power supply, checking how it all worked and noted the voltage thresholds prior to installing my Renogy unit. I wanted to make sure it would work well prior to installing.

Dutch061 · June 2021

10 gauge wire will safely handle 40 amps all day every day unless it is an extremely long distance. On your vehicle, you will be more than fine

Brad

dhauf · June 2021

Dutch061 said:

10 gauge wire will safely handle 40 amps all day every day unless it is an extremely long distance. On your vehicle, you will be more than fine

Brad

Technically though #10 wire is only rated for 30 amps. As you mentioned you can get away with short runs at 40amps but I personally wouldn’t push it.

Dutch061 · June 2021

@dhauf, my mistake I was looking at solid core and not multi-strand

Edit - I was on the complete wrong table, for 12 volts and up to 20 feet, it is rate at 46 amps based on 1/2 volt drop over the distance. Regardless, 20 amps will not be an issue.

Brad

dhauf · June 2021

Dutch061 said:

@dhauf, my mistake I was looking at solid core and not multi-strand

But honestly you are probably correct, I am sure you can get away with 40a in very short distances in a pinch.

m_lewis · July 2021

I’d like to take a moment to clear up another misconception. A DC to DC converter takes as an input some DC voltage and outputs a regulated DC voltage. Like all electronic devices, they are less than 100% efficient. So, if the output is 13.8 V at 20A, or 276 watts, then the input currrnt at an input voltage of 13.8 would have to be somewhat more than 20 amps. With top efficiency in gene range of 90%, that means at least 22 amps input.

If the output voltage is higher than the input, then even more current must be drawn. Example: 14.2 V @ 20 amps out = 284 watts. With 90% efficiency, an input of 316 watts is required. Given some voltage drop in the input wiring, let’s say a very conservative 0.5V, the input voltage would be about 13.3. 316 / 13.3 = 23.8 Amps. If the voltage drop is larger (due to small gauge or long wire, the input amp draw is even larger.

Real world values are likely to be closer to 25 amps in for 20 amps out.

LiFePO batteries will readily accept large amounts of current up until they are fully charged. So if your DC-DC converter is programmed to supply 14.2 volts, it will probably max out its current capability until the battery reaches that voltage.

To get back to the question of whether 10ga wire is adequate, the answer is probably. Just keep in mind that every connection, plug, crimp, wire, fuse, etc has resistance and thus voltage drop when current passes thru it; the more current, the higher the drop. Add these all up and you could easily see 1 or even 2 volts gone at 25 amps, which of course drives up the amps needed even further. It becomes a vicious circle as the wires heat up from their resistance. So make good connections, use big conductors, and keep wires as short as possible.

dhauf · July 2021

m_lewis said:

I’d like to take a moment to clear up another misconception. A DC to DC converter takes as an input some DC voltage and outputs a regulated DC voltage. Like all electronic devices, they are less than 100% efficient. So, if the output is 13.8 V at 20A, or 276 watts, then the input currrnt at an input voltage of 13.8 would have to be somewhat more than 20 amps. With top efficiency in gene range of 90%, that means at least 22 amps input.
If the output voltage is higher than the input, then even more current must be drawn. Example: 14.2 V @ 20 amps out = 284 watts. With 90% efficiency, an input of 316 watts is required. Given some voltage drop in the input wiring, let’s say a very conservative 0.5V, the input voltage would be about 13.3. 316 / 13.3 = 23.8 Amps. If the voltage drop is larger (due to small gauge or long wire, the input amp draw is even larger.
Real world values are likely to be closer to 25 amps in for 20 amps out.
LiFePO batteries will readily accept large amounts of current up until they are fully charged. So if your DC-DC converter is programmed to supply 14.2 volts, it will probably max out its current capability until the battery reaches that voltage.
To get back to the question of whether 10ga wire is adequate, the answer is probably. Just keep in mind that every connection, plug, crimp, wire, fuse, etc has resistance and thus voltage drop when current passes thru it; the more current, the higher the drop. Add these all up and you could easily see 1 or even 2 volts gone at 25 amps, which of course drives up the amps needed even further. It becomes a vicious circle as the wires heat up from their resistance. So make good connections, use big conductors, and keep wires as short as possible.

You're 100% right about the efficiency, my previous post was not entirely correct (thanks for clearing that up) but it's relatively close enough to the input amperage, generally they are around 80% efficient. Most dc to dc converters though will not draw more power than their rating. For instance Renogy states 20amps on the model I purchased with a max output power rating of 250W. If I have the voltage set to 14.6V and its consuming the full 20amps, that means I would get a max output current of 17.12amps to the battery. Almost every manufacturer includes a spec sheet which either give the efficiency rating and max input amps, or max output power rating which may differ on brand.

m_lewis · July 2021

Just checked my Victron Orion TR Smart 12-12 30 manual. It does not state maximum input amps.

It does claim 87% efficiency, a constant 30 amp output with a max of 40 amps at 25C.

Also states the typical amp draw is 35 amps. However, it gives no parameters for those values (output voltage, input vs output voltage). It does recommend 60 amp fuses on the input and output.

Output voltage regulation is only guaranteed down to an input voltage of 10 volts, higher if the output is set above 12.2 V. Doing the math, an output of 14.6 volts at 30 amps is about 100 watts over its 360 watt rating and would draw about 41 amps from the input.

I recommend 6ga input wire from the converter to the TV’s battery or alternator, along with a similar ground conductor. This will ensure minimum voltage drop and lower current draw for the same output. As a bare minimum, 8ga everywhere except the trailer-TV harness. Big short wires, few but strong connections.

dhauf · July 2021

m_lewis said:

Just checked my Victron Orion TR Smart 12-12 30 manual. It does not state maximum input amps.
It does claim 87% efficiency, a constant 30 amp output with a max of 40 amps at 25C.
Also states the typical amp draw is 35 amps. However, it gives no parameters for those values (output voltage, input vs output voltage). It does recommend 60 amp fuses on the input and output.
Output voltage regulation is only guaranteed down to an input voltage of 10 volts, higher if the output is set above 12.2 V. Doing the math, an output of 14.6 volts at 30 amps is about 100 watts over its 360 watt rating and would draw about 41 amps from the input.
I recommend 6ga input wire from the converter to the TV’s battery or alternator, along with a similar ground conductor. This will ensure minimum voltage drop and lower current draw for the same output. As a bare minimum, 8ga everywhere except the trailer-TV harness. Big short wires, few but strong connections.

Yeah I am not sure about Victron, and possibly some might draw more? I am not entirely sure. My Renogy also states 86% efficiency as well, but also has a 250w max power output for my 20A version. I'm gonna defer to you on your expertise with the Victron unit cause it might be a little bit different than mine. It all sounds good to me though.

Mouseketab · July 2021

I'm currently working on gathering the parts and bits to install the 20 Amp Renogy in my new F-150. I got the unit on Prime day for about 95, but all the wiring, connectors, etc are costing more than the unit. I'll post pics of my install when I get it done. I'm not interested in boondocking power, but just to make sure I can run the 12V fridge while on the road, and have decent battery power for my Power Mover when I arrive at my destination.

dhauf · July 2021

@Mouseketab That's a great deal! I know you are so right about the wiring and good quality connectors costing more. The price of copper has gone up quite a bit, I also ordered the stranded copper wire that has the silicone sheathing since it's so nice and flexible and the crimp connectors that have the heat shrink on them already, and it wasn't cheap. The 20A Renogy will do everything you are asking of it. I did a current test on the fridge and at 13.6v which is what my Renogy LiFePO4 battery was at fully charged the fridge pulled about 6amps while on battery, so that should leave you with plenty of headroom to keep the battery topped off and even run a couple of other things while driving. I am curious about your power mover. Did you install that yourself? I have a rather tight spot I park mine in when I bring it home and wonder if this might be a good solution to moving the camper around. How easy is it to install and which brand did you go with? Would love to see a pic of it if not too inconvenient.

Mouseketab · July 2021

dhauf said:

@Mouseketab That's a great deal! I know you are so right about the wiring and good quality connectors costing more. The price of copper has gone up quite a bit, I also ordered the stranded copper wire that has the silicone sheathing since it's so nice and flexible and the crimp connectors that have the heat shrink on them already, and it wasn't cheap. The 20A Renogy will do everything you are asking of it. I did a current test on the fridge and at 13.6v which is what my Renogy LiFePO4 battery was at fully charged the fridge pulled about 6amps while on battery, so that should leave you with plenty of headroom to keep the battery topped off and even run a couple of other things while driving. I am curious about your power mover. Did you install that yourself? I have a rather tight spot I park mine in when I bring it home and wonder if this might be a good solution to moving the camper around. How easy is it to install and which brand did you go with? Would love to see a pic of it if not too inconvenient.

I've got the Enduro Power Mover that I bought from Purplelineusa.com. Yes, I installed it myself with help from my pop. It took us about 8 hours over 2 days. It is really pretty straightforward to install. Next time I have a chance, I'll try and take a couple of pics of the mover. I've had it for 7 or 8 years, and it really works well.

Mouseketab · July 2021

I've got another question about the DC-DC charger. We are planning on totally bypassing the 7-way and connecting the unit to the trailer with Anderson connectors. The charger will be mounted in the truck with heavy wire going to an Anderson connector in the back, then routing some heavy wire from the camper battery and zip tying it along the 7-way cable to anderson connector.

Should we disable the "charging" line on the 7-way, or just leave it? The truck has a standard charge line with isolation relay in the 7-way.

dhauf · July 2021

Mouseketab said:

I've got another question about the DC-DC charger. We are planning on totally bypassing the 7-way and connecting the unit to the trailer with Anderson connectors. The charger will be mounted in the truck with heavy wire going to an Anderson connector in the back, then routing some heavy wire from the camper battery and zip tying it along the 7-way cable to anderson connector.

Should we disable the "charging" line on the 7-way, or just leave it? The truck has a standard charge line with isolation relay in the 7-way.

I personally would disable the other charge line if you’re running a dedicated line with Anderson connectors. You can access it from the small black box under the tongue and remove that wire from the terminal block and put some electrical tape on it. I believe it’s a black wire, but you can verify by putting a volt meter on it to check.

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