DC-DC capacity vs alternator capacity

[JonMN]

I have a 250 amp alternator in my Ford Transit based Coachmen Beyond. This feeds the coach AGM battery via a 150 amp fuse from the access point.

At the moment I have a BIM-160 that handles charging. I am considering replacing the 330AH AGM with a 300AH lithium.

The question is this: What would be a safe DC-DC maximum if I did not want to overly stress the alternator? I have searched quite a bit for opinions, and they run the gamut from just 30 amp to 60% of your alternator capacity, which would be a whopping 150 amp.

Here is my thinking: My coach air conditioning is a DC unit. I measured the draw at it's in the range of 64 amps when the compressor is running. On hot roads, it's nice to be able to run the coach air as well as the chassis air. I also have a DC refrigerator, but that takes less than 5 amps. So I was hoping to use a DC-DC of at least 70 amps so that I would not be losing ground with the coach air conditioner on.

Does 70 amp DC-DC sound safe? Would 100 amp DC-DC be out of line given the 250 amp alternator?

[hbn7hj]

What max capacity DC-DC chargers are available? I’m only familiar with 50 amp. I use 40 amp.

[JonMN]

Originally Posted by hbn7hj

What max capacity DC-DC chargers are available? I’m only familiar with 50 amp. I use 40 amp.

Thanks for the reply! Sterling makes units up to 120 amp. I was looking at their 70 amp and their 120 amp. They also allow you to regulate output to a degree, so their 120 amp could be limited to 100 by changing a setting.

[hbn7hj]

The limiting factor is alternator temp. Booster is the authority.

Get the big charger and dial it back as needed, I guess.

[phantomjock]

Originally Posted by JonMN

Thanks for the reply! Sterling makes units up to 120 amp. I was looking at their 70 amp and their 120 amp. They also allow you to regulate output to a degree, so their 120 amp could be limited to 100 by changing a setting.

Sterling also build a "mammoth" 200amp B2B, the BB1200. But it is NOT Cheap!

Your thinking that the alternator output must exceed the B2B is in my opinion correct. I came to that after having a number of interesting email "conversations" with Ben Sterling at the UK home of Sterling. If you are only going to the 120 amp size it is (last I checked) available in the US site, larger is not.

The alternator output at your desired operating range is important. So if you want to charge in the campsite, make sure your "idle" RPM alternator output is "up to the output" demanded by the B2B. Your 250amp "might" be sufficient. You'll need to check the specs.

I suppose you've checked out the instructions and info on their page:
https://cdn.shopify.com/s/files/1/06...f?v=1689077405

Cheers - Jim

[@Michael]

Originally Posted by JonMN

Does 70 amp DC-DC sound safe? Would 100 amp DC-DC be out of line given the 250 amp alternator?

The Ford Transit USA forum has threads on this topic, as does the Ford Body Builders manual (BEMM).

Some are reporting no issues at 120A. I'm comfortable with my current setup - just under 80A.

It also seems to depend on where you're tapping the power from. On mine, it's direct from the chassis battery.

Originally Posted by phantomjock

The alternator output at your desired operating range is important. So if you want to charge in the campsite, make sure your "idle" RPM alternator output is "up to the output" demanded by the B2B. Your 250amp "might" be sufficient. You'll need to check the specs.

Ford has an alternator output vs. RPM curve in their BEMM. At idle the 250A alternator spins at 2000RPM and delivers between 130 and 170 amps, depending on temperature.

[booster]

I think 70 amps is totally safe for your alternator, even by the time you add the power it has to make for running the van stuff like headlights, heater fan, ect, and the increased amp draw the engine will take when working hard like climbing a mountain.


As was mentioned, check the temp on the alternator after 20 minutes or so of idling or fast idling at the 70 amps, once you get it in place. Get a clamp on ammeter so you can actually measure the amps to the coach. Opinions vary on max allowable alternator temp and where to take that temp. I use 225*F and measure at the hottest spot, which on our alternators is the laminated stack between the two aluminum castings and also measure at the 12v output connection. The output is where many alternator temp controls read it, like the Balmar regulator.


Take any and all statements that "I have had no issues at 120 amps" with a grain of salt. Unless they have measured temps they really don't have any idea if they are causing any long term damage and heat damage is usually a longer term failure of shortened life. You have to get very very hot to fail in the short term. Be sure to also check it at full output after driving that way about a half hour with a totally hot engine like in stop and go driving. Under those conditions you could be looking at nearly 200* air coming through the radiator and if they don't have any fresh air guided into the the alternator it won't cool well with air that hot.

[JonMN]

Thanks all for the replies. This helps a lot!

In some regards the DC-DC might end up reducing the load on the alternator, as the BIM-160 lets the coach draw as much as it wants. So in the situation where the battery needs charging and I have the air conditioning on the draw might be pretty high.

With our use, there are only a few situations where we might be driving with a low coach battery AND the air conditioning on. So I was considering kind of the worst-case scenario.

[Davydd]

For those with Sprinters, I think the question is moot for Mercedes Benz Sprinters. The standard 180 amp engine alternator allows only 40 amps (per MB guidelines) for the house batteries. I don't know about the optional 220 amp alternator. That is why a second alternator devoted to the house batteries is preferred especially with high battery capacity. My second alternator some refer to it as an under hood generator is capable of outputting 280 amps driving after a short period of initial <330 amps and 220 amps on high idle.

With a 300ah lithium battery you might get by with the engine alternator but would still have to drive all day (8 hours) to replenish the batteries at a Sprnter 40 amp draw. That's impractical, not joyful and why its moot. With 800ah of lithium batteries forget it. We can drive a little over a half hour to replenish one day of house battery use (usually a maximum of 170ah) with the second alternator or 3/4 hours high idling. With our 576ah batteries with 460ah (80%) available we can stay in one place 2-3/4 days and replenish driving 2 hours and 40 minutes.

We have laid out a plan to go to the UP of Michigan in September with no stay over 2 days. We are mostly boon docking so can make the trip without hooking up to shore power. MB makes a recommendation not to idle over 2 hours without driving 40 minutes to blow out the particulate filter. I imagine you could cheat on that recommendation and idle 2-1/4 hours and get 3 days sitting in one place or just use electricity more judiciously. We usually don't worry about electrical use though we have a no propane van. We generally keep our inverter on to power, for convenience, to keep the coffee maker, induction cooktop and 115v outlets in use like we have shore power.

That's why I like the second alternator:

- No concern about the use of the engine alternator as in this thread.
- No need for propane.
- No need for an Onan type generator.
- Full use of electrical as same as shore power.
- Generally not concerned with electrical use.
- Faster battery charging.
- Unlimited lithium battery capacity. Space would be the limit.
- We actually have stayed in one place boon docking 5 days combined with battery capacity and idling on 576ah of batteries.
- Historically ARV installed 800ah of lithium on most vans as I had in my previous ARV van.
- We don't have practical air conditioning overnight but ARV is putting in a 48v (not Volta) high battery capacity solution to do so now I think. I don't know the details.

[MarcTremblay]

Originally Posted by JonMN

I have a 250 amp alternator in my Ford Transit based Coachmen Beyond. This feeds the coach AGM battery via a 150 amp fuse from the access point.

At the moment I have a BIM-160 that handles charging. I am considering replacing the 330AH AGM with a 300AH lithium.

The question is this: What would be a safe DC-DC maximum if I did not want to overly stress the alternator? I have searched quite a bit for opinions, and they run the gamut from just 30 amp to 60% of your alternator capacity, which would be a whopping 150 amp.

Here is my thinking: My coach air conditioning is a DC unit. I measured the draw at it's in the range of 64 amps when the compressor is running. On hot roads, it's nice to be able to run the coach air as well as the chassis air. I also have a DC refrigerator, but that takes less than 5 amps. So I was hoping to use a DC-DC of at least 70 amps so that I would not be losing ground with the coach air conditioner on.

Does 70 amp DC-DC sound safe? Would 100 amp DC-DC be out of line given the 250 amp alternator?

Went to 200ah lithium. Replaced bim-160 with Renogy dc2dc 50amps. Safe for alternator and batteries. Don’t forget to replace the wfco for a lithium charger. Went for a 50 amps there as well. Drawing more current from the alternator may make it overheat and wear prematurely. So I read.

[JonMN]

^^^ thanks everyone!