Toad battery charging

[BigTexEd]

Hello,

Background:
I'm installing/wiring up my Toad/Dinghy/Towed vehicle with all the necessary equipment. I am using a permanently installed brake controller. All supplemental brake controllers require power to work. The system I am using boosts the vacuum and utilizes an installed brake cylinder to apply the brakes using a proportional control.

Problem statement:
I have to disable the toad battery while under tow because the key needs to be in the accessory position to free up the steering wheel. I am using a solenoid to disable the battery while under tow. The only thing attached to the battery, under tow, will be the supplemental braking system. I want to ensure the battery stays charged while under tow. If the battery gets too low due to use, vacuum leak, or battery performance, the supplemental braking system will not work and the safety of the rig is compromised.

Issues with existing solutions:
1) Using the charging wire (#4) from the 7 pin trailer connection. The MB Sprinter chassis manual says to NOT use this for charging the battery under tow. I can see multiple problems with this. a) Manual says not to use it. b) It is not regulated. c) No protection for anything (under volt, over volt, short circuit). This can/will lead to damage to the toad battery, coach wiring, or anything in between. This brute force method is fraught with problems.

2) Roadmaster charge line kit. this does have inline breakers which is better than using the wire from the 7 pin, but 15A continue to flow to the toad battery without any regulation. This means that the toad battery could continue to charge after full, and this is bad. This is like using a battery charger that is not automatic.

3) RVI towed battery charger. This is better than the 7 pin version and the Roadmaster. This does have voltage regulation. This uses the 7 pin input "hot" connection as an input, but then regulates it to charge the battery. There is feedback to it's operating value. However, I don't know how it works. Does it simply transfer the charge from the coach to the toad? In other words, the current from the 7 pin connector is just a few amps. You need to boost the current to charge the battery to around 20-30A.

What I really want is a DC to DC boost charger that has full regulation and temperature control. What is the BEST solution out there?

[Winterbagoal]

Just use the hot pin (#4) in the 7 pin connector to charge your towed's battery.

[BigTexEd]

Originally Posted by Winterbagoal

Just use the hot pin (#4) in the 7 pin connector to charge your towed's battery.

OMG. No... I responded about why this is a bad idea on many levels in this post and many others.

[BobC]

I don't know about which DC to DC charger is the best, but Renogy's DC to DC charger has received a good review from Will Prowse, a generally well-respected pod-caster. Some of the smarter (and more expensive chargers) have more flexibility in terms of charge profiles, etc., which you probably don't need in order to charge your toad battery:

https://www.youtube.com/watch?v=EcVjPap9dkY

I installed one in the Econoline van I'm converting to charge the house batteries and it's working well.

https://www.renogy.com/12v-dc-to-dc-...ttery-charger/

Currently, the 20A version is only $108 via Amazon and $110 via Renogy, not significantly more than RVI's Towed Battery charger+ ($95). I have an RVI unit on my toad but I'd rather have the Renogy charger.

[DavidM]

Originally Posted by BigTexEd

Issues with existing solutions:
1) Using the charging wire (#4) from the 7 pin trailer connection. The MB Sprinter chassis manual says to NOT use this for charging the battery under tow. I can see multiple problems with this. a) Manual says not to use it. b) It is not regulated. c) No protection for anything (under volt, over volt, short circuit). This can/will lead to damage to the toad battery, coach wiring, or anything in between. This brute force method is fraught with problems.

This is what I would do except for the MB warning which I don't understand. Pin #4 on most vehicles is fused at 20-30 amps and is powered by a relay that shuts off when the ignition is off. It will charge a T/T battery at 5-10 amps which is somewhat regulated by the voltage drop in the long wire from the TV's battery. It starts at the fixed voltage set by the internal regulator on the TV's alternator- 13.5-14.0 V and drops a half a volt or more at the T/T's battery . If it can't hurt the TV's battery I can't see how it will hurt the Toad's battery, particularly at a half volt lower.

I would do some more investigating into why MB says not to use #4 and then try to mitigate those reasons. It is by far the simplest solution.

2) Roadmaster charge line kit. this does have inline breakers which is better than using the wire from the 7 pin, but 15A continue to flow to the toad battery without any regulation. This means that the toad battery could continue to charge after full, and this is bad. This is like using a battery charger that is not automatic.

3) RVI towed battery charger. This is better than the 7 pin version and the Roadmaster. This does have voltage regulation. This uses the 7 pin input "hot" connection as an input, but then regulates it to charge the battery. There is feedback to it's operating value. However, I don't know how it works. Does it simply transfer the charge from the coach to the toad? In other words, the current from the 7 pin connector is just a few amps. You need to boost the current to charge the battery to around 20-30A.

Two and three are both unneeded complications.

What I really want is a DC to DC boost charger that has full regulation and temperature control. What is the BEST solution out there?

If this is still your preferred solution, Victron, Redarc and Renogy make DC to DC chargers that have three step charging alorithims. I think one of them makes a 20A version which should not tax pin #4. But it is totally unneeded unless there is something different about the Mercedes pin #4. And if there is, then many T/T owners who tow with a Sprinter or even MB Bluetec sedans will be disappointed. Probably not many of those though.

David

[BigTexEd]

Originally Posted by DavidM

If this is still your preferred solution, Victron, Redarc and Renogy make DC to DC chargers that have three step charging alorithims. I think one of them makes a 20A version which should not tax pin #4. But it is totally unneeded unless there is something different about the Mercedes pin #4. And if there is, then many T/T owners who tow with a Sprinter or even MB Bluetec sedans will be disappointed. Probably not many of those though.

David

Thank you for all this information! I spent some time researching them.

The charge in the #4 pin is around 3A. The wire is super thin. I measured it. It’s not 10A. Maybe it depends on how much alternator is supplying at a given moment. I can assure you I haven’t seen 10A. I do agree that the voltage regulation is through the resistance of the long wire which connects along the coach to the connector. It keeps it safe. Funny that this is the regulation. The equivalent would be a non-automatic motorcycle battery charger in your garage with a wire 25 feet long where the charger is unregulated. It’s primitive, but works. Barely. For all the details people put into their coach, this is a caveman approach to charging, but it does have a modicum of utility.

The DC to DC chargers that I have seen out there don’t have input current requirements. That is critical. They assume a supply battery. The #4 pin charge is not equivalent to this supply battery.

The RVI has potential. I bet you it would take low input current. I’m going to call them.

I was hoping someone would have figured all this out. The current approach of using #4 is the primary charging works by accident or barely works at all. The MB chassis manual says it and I’ll bet you it’s the same reason I’m saying. The reason why the market doesn’t have an elegant solution is because people think they don’t need because they just plug in #4 to their battery and think it’s charging. It’s not. It’s maintaining .... barely...and it violates what the chassis manual says.

[DavidM]

Three amps is low I agree, but maybe the toad's battery is fully charged and at a voltage high enough to prevent much current flowing.

I have measured this pin #4 current in two cases. One was when the T/T's battery was discharged to about 50%. It measured 10 amps. Then when the battery was fully charged it dropped to 4-5 amps.

What size is the wire to/from pin #4, both from the MB and to the toad's battery? It should be at least #10.

David

[l1v3fr33ord1]

BigTexEd-

If the chassis maker supplies a wire between the chassis battery and the hitch connector, it is usually of sufficient gauge and with a fuse as close to the battery as possible. The wire may or may not be ignition-switched. Voltage regulation and a diode in the line are optional, and some choose to use install or both. The alternator will charge the two batteries as if they are one. This works well.

But you have a Sprinter, and you say that they say not to use the charge line. I wonder why they would include something, then tell you not to use it?

If I couldn't determine the actual "why" behind that warning, and evaluate myself whether it applied in this case, then I might consider replacing the Sprinter's wire with one of my own, run from the chassis battery to the hitch connector, of sufficient gauge (I'd recommend 12 or better yet, 10 AWG) and fused as close to the battery as possible. This is a common "fix" for those coaches that don't come with a working charge line. Make sure the umbilical cable and the toad end wire gauge is sufficient as well. Finally, place a fuse in the toad end of the charge line, as close to the battery as possible.

I'd bet that the current draw of the permanently-installed system is low enough that you could get by with what is in the Sprinter already. But, you'd have to make peace with the warning in the manual. I suspect that MB is trying to protect that "super thin" wire, or the alternator, from too much charging current, as could occur if the toad battery was close to dead. I suppose only MB knows why the warning is there.

One question: Is your Sprinter's chassis battery lead-acid or lithium?

[creativepart]

While some folks here my well know the answers to your questions, I bet the https://sprinter-source.com/forums/index.php would be an additional source of good info on this.

[DavidM]

I totally agree with Creativepart's postings above. But I don't understand his question about the MB having a lithium battery or lead acid. Aren't all starting chassis batteries lead acid and if you tried to install a lithium battery, wouldn't the BMS limit the current and it probably wouldn't start?

FWIW I have measured starting current on my 3.5 liter gasoline engine. It takes 200+ amps. A diesel like the MB's would take even more due to the higher compression.

David

[BigTexEd]

Originally Posted by l1v3fr33ord1

BigTexEd-

If the chassis maker supplies a wire between the chassis battery and the hitch connector, it is usually of sufficient gauge and with a fuse as close to the battery as possible. The wire may or may not be ignition-switched. Voltage regulation and a diode in the line are optional, and some choose to use install or both. The alternator will charge the two batteries as if they are one. This works well.

But you have a Sprinter, and you say that they say not to use the charge line. I wonder why they would include something, then tell you not to use it?

If I couldn't determine the actual "why" behind that warning, and evaluate myself whether it applied in this case, then I might consider replacing the Sprinter's wire with one of my own, run from the chassis battery to the hitch connector, of sufficient gauge (I'd recommend 12 or better yet, 10 AWG) and fused as close to the battery as possible. This is a common "fix" for those coaches that don't come with a working charge line. Make sure the umbilical cable and the toad end wire gauge is sufficient as well. Finally, place a fuse in the toad end of the charge line, as close to the battery as possible.

I'd bet that the current draw of the permanently-installed system is low enough that you could get by with what is in the Sprinter already. But, you'd have to make peace with the warning in the manual. I suspect that MB is trying to protect that "super thin" wire, or the alternator, from too much charging current, as could occur if the toad battery was close to dead. I suppose only MB knows why the warning is there.

One question: Is your Sprinter's chassis battery lead-acid or lithium?

The name of the pin for #4 is “+”. It doesn’t say charge. That is the big difference. I’m here to debunk this popular method no matter it’s adoption rate. People assume it’s ‘charge’ but it is just a nominal + voltage and supply. The wire on the MB chassis is woefully small. The wire on my Tahoe and Colorado is about 3x the size.

The chassis battery is not lithium. I think it’s lead acid. It could be AGM.

[BigTexEd]

I found this in the toad section of the FAQs. It highlights some of the same issue.

[DavidM]

So given the facts presented: 1) MB says not to use pin #4 for charging, 2) the wire is small, and 3) you only measure 3 amps when connected to the toad's battery, I finally throw in the towel and support creativepart's suggestion to install a separate #10 gauge circuit (pos and neg) from the MB's battery to the hitch connector. Fuse it near both batteries. Make sure the new circuit includes the negative because I am sensing that the MB's connector is all small wire. You need a decent size negative to supply 10 amps or so.

Apparently MB's connector is only good for powering the trailer lights and probably only led trailer lights if it is something like 16 gauge wire or less and only provides 3 amps. So don't try to make that work with a separate DC to DC charger or similar.

FWIW 16 gauge with 3 amps on a 25' circuit causes a voltage drop of 0.6 volts. 10 amps would be almost 2 volts.

Creativepart's suggestion to check with Sprinter-Source is also a good one. Ask you question in the appropriate chassis forum given your chassis model year and realize that it is often a year less than you coach.

David

[creativepart]

Originally Posted by DavidM

I finally throw in the towel and support creativepart's suggestion to install a separate #10 gauge circuit (pos and neg) from the MB's battery to the hitch connector.

David, you have your poster's confused. I only suggested checking the Sprinter-Forum for more info. I'm not sure who suggested what you're attributing to me.

I'm with you though, if you tow a travel trailer with a Sprinter you are going to be charging the TT's battery(ies) via that #4 pin of the 7-pin connector. So, I fail to comprehend the difference with a Toad.

But I would check the Sprinter-Forum. This is right up their bailey-wick, so to speak.

[BigTexEd]

Originally Posted by creativepart

David, you have your poster's confused. I only suggested checking the Sprinter-Forum for more info. I'm not sure who suggested what you're attributing to me.

I'm with you though, if you tow a travel trailer with a Sprinter you are going to be charging the TT's battery(ies) via that #4 pin of the 7-pin connector. So, I fail to comprehend the difference with a Toad.

But I would check the Sprinter-Forum. This is right up their bailey-wick, so to speak.

A trailer has a very small battery. It’s for the breakaway to ensure the brakes turn on. For that purpose, the #4 wire will be perfect for it. It’s small, low current, and has little power draw. That is the main purpose for #4. Trailer manufacturers have also used this for various led lights which take away from the safety battery, but still ok in my opinion as long as not many lights stay on.

A toad has a deep cycle, big battery. That is the main difference. What people use for the trailer (correctly), people with toads are using to charge their big, deep cycle battery. It’s not for that. It never was meant for that. It barely works. That is why third party, robust manufacturers have you run a separate line to the coach battery.

I do believe a separate wire all the way back to the coach battery with thick wires and auto fuse protected is the best choice for a given amount of money. It’s easy, effective, and provides voltage regulation in the device which is safe.

What I’m trying to do with a DC to DC boost charger is take a few milliamperes and boost it to 25A. Using this technique, you can use the + #4 connector to supply a little bit of current to boost to a lot of current. This is what I do in my vocation, but in a chip. The big difference here is that I don’t need to run the extra wires across the coach and across the hitch and into the toad. Furthermore, I can make the switching automatic. I am researching this and it does look plausible.

[creativepart]

Originally Posted by BigTexEd

A trailer has a very small battery. It’s for the breakaway to ensure the brakes turn on.

That's where you'd be misinformed. Travel Trailers have as much as 200ah of deep cycle batteries riding on the A-Frame to power the 12v needs of the trailer. Yes, it powers the emergency breakaway switch, too. But nearly all RV Travel Trailer owners rely on their 7-way to charge up their house batteries as they pull the trailers to and from campsites.

[Elkman]

Using the trailer connector to charge the house batteries entails using a 10 gauge wire for the hot lead as more current is needed than if only the running lights need power. It is also important to put in a relay so the trailer does not drain the tow vehicles batteries when the engine is off.

If a battery in a toad cannot maintain adequate voltage while it is being towed then it is time to replace its battery. At a minimum go to a shop where they can use a load tester to check out the battery that you have in it now.

For on the road use a small 6 amp battery charger and a long extension cord can be used to insure the battery is maintained, especially during very cold weather as it protects the battery.

[BigTexEd]

Originally Posted by creativepart

That's where you'd be misinformed.

Please don’t assume or tell me I’m uninformed. I don’t do that to you or anyone else. It’s a courtesy we should all give to each other. We are just discussing the merits of all these techniques. My analysis is rooted in engineering and facts. I have provided many documents and supporting images from various sites. You haven’t done any of that.

[DavidM]

Creativepart, did I screw up again and attribute something to you incorrectly? Again I apologize.

There are several misunderstandings articulated below:

Originally Posted by BigTexEd

A trailer has a very small battery. It’s for the breakaway to ensure the brakes turn on. For that purpose, the #4 wire will be perfect for it. It’s small, low current, and has little power draw. That is the main purpose for #4. Trailer manufacturers have also used this for various led lights which take away from the safety battery, but still ok in my opinion as long as not many lights stay on.

A toad has a deep cycle, big battery. That is the main difference. What people use for the trailer (correctly), people with toads are using to charge their big, deep cycle battery. It’s not for that. It never was meant for that. It barely works. That is why third party, robust manufacturers have you run a separate line to the coach battery.

As creativepart notes, T/T batteries can be big and deep cycle. A toad starting battery is not deep cycle, it is a starting battery and is usually 500+ CCAs and 60-80 amp hours.

I do believe a separate wire all the way back to the coach battery with thick wires and auto fuse protected is the best choice for a given amount of money. It’s easy, effective, and provides voltage regulation in the device which is safe.

What I’m trying to do with a DC to DC boost charger is take a few milliamperes and boost it to 25A. Using this technique, you can use the + #4 connector to supply a little bit of current to boost to a lot of current. This is what I do in my vocation, but in a chip. The big difference here is that I don’t need to run the extra wires across the coach and across the hitch and into the toad. Furthermore, I can make the switching automatic. I am researching this and it does look plausible.

Wow!! You are trying to boost the 3A you get now from pin #4 to 25A with a DC to DC converter. No way if the wire is small as you say. You would never get 25A though #16 gauge and if you did you would exceed its ampacity

[creativepart]

Originally Posted by BigTexEd

Please don’t assume or tell me I’m uninformed.

Originally Posted by BigTexEd

A trailer has a very small battery. It’s for the breakaway to ensure the brakes turn on.

Which may be true for some utility trailers, but not for most modern travel trailers.

I'm not trying to argue or create a confrontation. Just stating that if you think travel trailers don't have significant house battery banks charged via the 7-pin that perhaps you are not aware that this is so.

I apologize if my choice of words offended you. That was not my intent.