switch/relay to reverse DC DC charger inputs --> charge starter battery

kite

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Question: Is it possible to use a manual transfer switch or relay to swap the input & output of a DC - DC charger, in order to charge a starter battery from a house battery? sketch attached that I hope shows what I mean

This could be accomplished with:
- a switch like this (most are AC rated...would this be ok for 50A 12V?)
- a couple automotive relays
- a single DPDT relay like this (though I don't love the idea of always charging through some no-name amazon relay)

And a way to manually switch the charger on (in normal operating config it will automatically sense engine running). This can be done with an L-H jumper, H pin > 3V, or L switched to ground.

Any obvious reasons this will not work? Better suggestions of hardware to accomplish this?


Why:
I'm switching my van's house battery to lifepo4. The current setup has lead acid house batteries:
- charged directly from the alternator via a Blue Sea Automatic Charge Relay (parallels starter and house banks when either > 13V)
- and manual battery switch that directly parallels the house and starter batteries, allowing me to jump start from the house batteries if needed

I plan to use a Victron Orion 12-12 30A non-isolated DC-DC charger to charge the new LFP battery from the alternator. I would love to retain some way of starting/charging the starter battery from the house bank if I accidentally kill the starter.

An alternative is to use a battery switch to parallel the house and starter banks (bypassing the DC DC charger) and have a less controlled charge. The OCD part of me likes the idea of using the existing charger...I don't mind waiting a little while for the starting batt to charge.
 

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jharrell

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Not a bad idea but probably easier to get a small starting maintainer like this: http://www.lslproducts.net/TLSPage.html going the other way and have a momentary booster switch with solenoid for emergency starting (Probably can wire the existing BlueSea as momentary if it has external switch control). Not sure if there would be any issues with a connection loop with two chargers and or solenoid engaged, don't think there would, I ran a amp-l-start with stock solenoid for a while.

I currently have a large Automatic BlueSea ACR with remote control switch for my AGM house but trying to figure out what I will need to change for going to LFP. My F53 gas motorhome has a 175 amp alternator and seems to have no issue supplying 80-90 amps continuous to the house but not sure how it would respond with low impedance discharged LFP's. I would like to just keep the ACR as the alternator put out about 14.1v back at the batteries and is simple and robust.

Wish someone would make a two way dc-dc charger.
 
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HRTKD

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Seems complicated to me. I keep a NOCO GeniusBoost 150 in my tow vehicle, connected to a 12v port. It's always ready and is an easier way to jump start another vehicle than breaking out the jumper cables.

Is this solution for when the RV is in storage?
 

jharrell

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If you keep your chassis lead acid battery charged they last at lot longer and if the RV isn't your primary vehicle yeah it becomes a storage issue. Also I had my chassis go dead due to a parasitic load while camping (think my laptop drained it off 12v plug in dash). If solar or shore power keeping chassis charged you don't have to think about it ever, nice upgrade for not much money, most motorhomes and boats have some sort of automatic charging relay to deal with this which is straight forward when the batteries are of the same chemistry.
 
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kite

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Thanks for the responses.

Wish someone would make a two way dc-dc charger.
Yeah, wouldn't that be nice!


To clarify, this is for a daily driver beach van + camper. I do not need a trickle charger/maintainer. The goal here is to be able to start the engine if I accidentally kill the starter battery at some remote location (say, 50 miles out dirt at a remote beach in Baja). I've used my battery combine functionality to start the engine from house bank exactly once since installing the electrical system in my van years ago and was very happy to have added this feature.

I like the idea of using the charger I'm installing to control charge. I doubt it would take long at 30A to boost the starter battery enough to start my smallish diesel.
Carrying a secondary jumper pack seems silly if I have a large LFP pack in the van...

A momentary boost switch is an option, it's just much less controlled. Current depends on voltage at both batteries, wire resistance, etc. Then if I crank the engine with them connected will I trip my 120A BMS? I don't know. Maybe it's totally fine and the simplest for the very very occasional use this will get.

Probably can wire the existing BlueSea as momentary if it has external switch control
Interesting thought. I believe is a 60A relay, so maybe could handle the current without cranking engine. I will check the manual and see if it can be manually switched, and if that will disable the automatic switching.
 

jharrell

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Yeah the BMS cutout could be an issue, if the chassis battery is very discharged as soon as you parallel with the LFP bank a large current will flow to the starter batt as large as the starter battery will accept which could be hundreds of amps, on top of that if you attempt to start the starter will pull a large quick surge then a good 100a continuous until engine starts. My BlueSea ACR is 500a rated so it can handle starting current or whatever. I was planning two LFP house batteries with 100a BMS so 200 continuous with probably 400 or more for a few seconds before cutout.

So perhaps just some good manual plugs to reverse charging that are accessible since it would be a rare occurrence if you really want to do a relay yeah would get a good one which could be pricy.

LFP have large capacity but their BMS's really limit their ability to jump start I think its mostly due to the mosfets used in the BMS, if it where contactors or bigger / more mosfets then they could provide starting currents without much issue.
 

eXodus

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Yeah the BMS cutout could be an issue, if the chassis battery is very discharged as soon as you parallel with the LFP bank a large current will flow to the starter batt as large as the starter battery will accept which could be hundreds of amps, on top of that if you attempt to start the starter will pull a large quick surge then a good 100a continuous until engine starts. My BlueSea ACR is 500a rated so it can handle starting current or whatever. I was planning two LFP house batteries with 100a BMS so 200 continuous with probably 400 or more for a few seconds before cutout.
It's a flooded lead battery - it will not accept hundreds of amps as charge. Discharge yes -hundreds or thousand amp.

As rule of thumb - a lead battery will accept about it's rated AH on A as charging current. A starter battery is 60-80AH - so it will not charge with more 60-80A (probably less when deep discharged)


Just hit the momentary switch and keep them connected for a few minutes before trying to start the engine - then the lead battery has the chance to charge and then does most of the heavy lifting and not tripping your BMS.


An alternative is to use a battery switch to parallel the house and starter banks (bypassing the DC DC charger) and have a less controlled charge. The OCD part of me likes the idea of using the existing charger...I don't mind waiting a little while for the starting batt to charge.
your picture is incomplete - the DC to DC charger is getting it's power from somewhere - and so you are potentially building a loop.

There are various ways of doing this.

I've got a Diode from the House to the Chassis battery - everytime the house battery is 0.3V above the chassis battery it's getting charged.
In your case that wouldn't work - because the DC to DC is detecting the higher voltage on the chassis battery and is starting to charge the house battery - so we just built a loop.
The same thing for DC to DC 12V battery maintainer.
Can you program then Orion DC to DC that it's only on with the engine?

How big of an alternator do you got and how big is the LFP is going to be?
If the alternator is large enough and not a massive LFP you may don't need to do anything. Just keep the existing stuff.
 

Short_Shot

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The loop issue is solved by cutting off the house battery charger when the starter battery charger is enabled. Easily done with a double throw relay.

The Orion can be set up to only run when alternator voltage is detected as well, but any dc charger setup should have a means to disable it when the vehicle isn't running of course.
 

jharrell

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It's a flooded lead battery - it will not accept hundreds of amps as charge. Discharge yes -hundreds or thousand amp.

As rule of thumb - a lead battery will accept about it's rated AH on A as charging current. A starter battery is 60-80AH - so it will not charge with more 60-80A (probably less when deep discharged)


Just hit the momentary switch and keep them connected for a few minutes before trying to start the engine - then the lead battery has the chance to charge and then does most of the heavy lifting and not tripping your BMS.
Don't think it was mentioned whether starting battery was AGM or Flooded, my AGM's will accept 5C if discharged. Either way a dead starting battery will have a much lower voltage than the LFP's which are very stiff, current will be based on the voltage difference and the starter battery IR which will change rapidly upon paralleling the question will be if its within the BMS cutout time, I think it will easily exceed 100amps at least for a short period upon connection but honestly can't seem to find good info on the IR of a dead flooded start battery.
 

A.Justice

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Don't forget you have to change the voltage profile for charging a lithium battery versus lead acid.

IMHO, the safest way to charge 3 sets of different chemistry batteries would be to use 2 separate DC-DC chargers.

Alternator --> starter battery --> DC-DC #1 --> battery 1.

Alternator --> starter battery --> DC-DC #2 --> battery 2.
 

eXodus

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Don't think it was mentioned whether starting battery was AGM or Flooded, my AGM's will accept 5C if discharged. Either way a dead starting battery will have a much lower voltage than the LFP's which are very stiff, current will be based on the voltage difference and the starter battery IR which will change rapidly upon paralleling the question will be if its within the BMS cutout time, I think it will easily exceed 100amps at least for a short period upon connection but honestly can't seem to find good info on the IR of a dead flooded start battery.
AGM is different. They are much better at accepting charge. Usually most Cars and Trucks/RVs have still flooded batteries as starter battery.

Flooded lead batteries are interesting creatures, they in general have a high internal resistance - which goes up when they are fully charged, but also when they get under a certain level of charge.

Those are two different chemical effects, when the battery is getting fully charge - the electrolyte is getting saturated - resistance goes up. That's your normal charge.
When you deep discharge - you get those lead crystals forming on the plates. They initially limit the charge until they dissolve again. Looks like a U curve. Which doesn't happen in AGMs. they don't form those crystals.

The fastest a Flooded lead charges, what I've seen is at around 50-60% state of charge - around 1C.
IMHO, the safest way to charge 3 sets of different chemistry batteries would be to use 2 separate DC-DC chargers.
would be also a expensive way.

Actually from people who actually try LFP + lead in parallel charging - I read that most actually complaining about the LFP not charging fast enough. Because the voltage from the alternator is to low (13.8V - 14.4V depending on the vehicle) to push large amps to LFP
 

kite

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In my case, starter battery is a flooded lead acid.
150A alternator
280Ah 12V lithium house battery

So the options being suggested are:
1) momentary boost switch that parallels house and starter around DC DC charger
  • pros:
    • simple, ish
  • cons:
    • uncontrolled current.
      • initial current spike might trigger BMS over current? Differing opinions here. If @eXodus is right about flooded lead acid charge acceptance rate, this is probably ok...but it's an unknown and not trivial to test.
      • fuse sizing not straight forward (likely greater than I would install if just sizing for the 30A Orion)
    • input and output of DC DC charger now connected, loop created if charger turns on. At a minimum, would need to disable the Orion automatic engine sense, and wire it to turn on when the ignition is on. Some complexity is added back in..

2) 2 x DC DC charger. NO, expensive and overkill

3) carry a lithium jumper pack. NO, expensive

4) reversing DC DC charger inputs. At this point this is my plan.
  • I like the suggestion from @jharrell about using some beefy plugs to reverse positive inputs on the rare occasion this is needed. These connectors look perfect and cheap. I can color code the normal input & output pairs (charging house bat), and reverse them to charge the starter battery.
  • I will still need to wire a jumper & switch to manually turn the charger on in the reverse use case. This is a problem I'll solve once I have it in hand and can assess all the options. Maybe I can just turn it on with bluetooth?
  • I'm not worried about the charge profile matching. Any LFP bulk charge voltage will be safe for a FLA, and it will be temporary.
 

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kite

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Works like a charm.

manually turning on the victron orion is super simple.
  • leave engine detection on with the jumper in (it has to be in normal operation).
  • supply >3V to the H-pin (but with jumper in this is either pin) and it turns on. So I just need to add a switched wire from my 12V bus bar to the jumper.
To charge the starter battery I will:
1: disconnect the powerpole plugs on the IN and OUT of the Orion
2: plug them in with in/out reversed (colors mismatched)
3: switch on 12V to the jumper

tested and functional, though I haven't added the switch yet.
 

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kite

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switch installed in a protected location so it doesn't get accidentally pressed. It all works as expected!
 

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