DC>DC charger AND isolator? Or instead of isolator?

[strollingbones]

Hi Gang! Newbie here and this is my first post. I have extensively RTFF and learned a lot. Thanks!

Quick Backstory
I have a Roadtrek Class B motorhome with a state of the art electrical system. State of the art for the year 2000, that is. So my Covid-19 project is to upgrade it all.

Currently, the alternator charges both the starter and the house batteries. There is an isolator that prevents the house electrical from draining the starter battery.

Now I am considering upgrading my batteries to LiFePO4, and I understand this system might not cut it anymore. Some experts believe LiFeP04s can draw so much current when charging that they damage the alternator. They advise installing a DC>DC charger, like this one, to regulate current draw from the alternator to the batteries.

My Questions
#1) Do I correctly understand the situation and the role of the DC>DC charger?

#2) If I install a DC>DC charger, do I remove the 20 year old battery isolator? Or do I use them both somehow?

Thanks in advance for your assistance!

 

[HRTKD]

The battery isolator in my ATV is rated for a car/truck. It is a one-way current once the main battery is charged (that's a weak description, but used for simplicity). Until the main is charged, the secondary battery doesn't see any charge current.

Should your starter battery die, the isolator should have a means to allow current to flow the opposite way, using your house batteries to charge the starter battery, letting you start the engine.

The DC-DC charger would go downstream of the isolator. However, the charger is only going to work one-way. If your starter battery died and you set the isolator to allow current to flow back to the starter battery, nothing would happen. This is because the charger doesn't charge both ways.

 

[grizzzman]

#1 Yes
#2 Yes

 

[Indolent58]

I am in the same process and pondering the same question. Most discussions I have seen talk about either/or not both. I don’t know what the consequences if any would be if both were used but the Renogy DC-DC charger has isolation built in. It takes a positive current from the vehicle ignition switch to start charging. So if the ignition is off no current is drawn from the starter battery. This is exactly what the stock relay isolator in my RV does except that when the Renogy is engaged current can only flow one way. As mentioned any “Battery boost” function (we have a switch on our dash) that would use house batteries to start the vehicle if the start battery is dead won’t work. Neither the Renogy nor my stock isolator care about the voltage of the starter battery so I suppose if it’s borderline the additional load of the charger could make starting even harder. Voltage Sensitive Relay (VSR) isolators behave as HRTKD describe - they only pass current to the house battery when the starter battery is fully charged, but I doubt that’s what you have now given the age of your RV.

 

[Hovanic]

I have a 2016 Class A. According to the documentation it has a BIM160 which makes no mention of Lithium.
Quick search found this LI-BIM160. Anyone on here using this? Looks like a direct replacement.

 

[Opusdei]

I have a second thought about the role of DC>DC charger. I consider LiFePO4 battery can replace any type of 12V batteries and that's what you are going to do with your project. Why bother to replace the isolator then? Keep in mind that I'm not a mechanic or electrical expert about alternator though.

 

[HRTKD]

I have a second thought about the role of DC>DC charger. I consider LiFePO4 battery can replace any type of 12V batteries and that's what you are going to do with your project. Why bother to replace the isolator then? Keep in mind that I'm not a mechanic or electrical expert about alternator though.

I assume you mean replace both the starter and house batteries with LiFePO4. Using LiFePO4 to start a V8 engine may not be the best use of that battery. Plus, the BMS may not be rated for the cranking amps needed.

 

[Freep]

I replaced my isolator with a Victron Orion TR 30. There are a couple of things I really like about it. One is that I can connect to it with my phone and check to see that it's working while on the road.

The other thing that has come in handy is putting it in power supply mode. I completely drained my battery one night and in the morning I let the truck batteries supply power for a little while before the sun came up.

 

[HaldorEE]

I have a second thought about the role of DC>DC charger. I consider LiFePO4 battery can replace any type of 12V batteries and that's what you are going to do with your project. Why bother to replace the isolator then? Keep in mind that I'm not a mechanic or electrical expert about alternator though.

The isolator is there so that running the house loads in the RV can't drain the starter batteries stranding you in the middle of nowhere. What the isolate will not do is protect your LiFePo4 house battery from being overcharged by the alternator. Nor will it prevent your house battery from cooking the alternator if the house batteries are at too low of a SOC when you try to charge them from the alternator.

A DC-DC charger (like an Victron Orion TR) will provide both charge management for your new lithium batteries and will also protect against draining the starter battery with house loads. The Orion will not however change your starter battery from the house battery.

Far as I can see the DC-DC converter plus a set of jumper cables (in the event you need to jump start the engine from the house battery) will do everything you need. I would replace the isolator with the DC-DC converter since otherwise you will get unnecessary power drop in the isolator.

An alternative if you will have solar power is to use a Renogy DDC50. The provides both alternator and solar charging of the house battery and will also divert solar power to charge the starter battery if needed. The downside to the DDC50 is it is 12V only (no 24V house battery), is not really configurable and has very a low solar panel input voltage limit (25V).

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping

www.renogy.com

There is an alternative to the Renogy with a higher solar input voltage limit, but the name escapes me. Starts with a K.

 

[HaldorEE]

Redarc also has an option, but I doubt if it is in your budget. I know it isn't in mine.

Battery Management Systems : REDARC 12V Systems

Level up your dual battery system with a REDARC battery management system. Manage house and auxiliary battery systems in your truck, trailer or RV.

redarcelectronics.com

 

[HaldorEE]

Found it. Kisae DMT1230. Max solar voltage limit is 45V so it will let you put two 12V panels in series or use one 24V panel. Not fantastic, but not 25V

KISAE DMT1230 Abso 30A DC-DC Battery Charger | DonRowe.com

The KISAE DMT1230 DC to DC Battery Charger is a multi-stage, multi-input battery charger capable of charging different types of batteries commonly installed in boats, RV's, 4WD's and commercial vehicles.

www.donrowe.com

This version will not charge the starter battery from solar like the Renogy will.

 

[Opusdei]

The isolator is there so that running the house loads in the RV can't drain the starter batteries stranding you in the middle of nowhere. What the isolate will not do is protect your LiFePo4 house battery from being overcharged by the alternator. Nor will it prevent your house battery from cooking the alternator if the house batteries are at too low of a SOC when you try to charge them from the alternator.

My alternator voltage is around 14V and house battery works fine with relay charging. I am wondering if there is a simple way to limit such overcharging.

 

[John Frum]

My alternator voltage is around 14V and house battery works fine with relay charging. I am wondering if there is a simple way to limit such overcharging.

3.65 volts per cell or 14.6 volts for the pack is maximum charge voltage.
In reality you don't need to go that high to fully charge a lifepo4 battery, it just takes longer.
Leaving a battery exposed to voltage beyond its resting full voltage is stressful for the battery.
The longer the duration the greater the stress.
The orthodox solution is to use a dc2dc charger that either disconnects when the battery is full or drops to a "float" voltage.
"float" for lifepo4 batteries can be understood to be power assist as in the charger is helping the battery with the loads.

 

[Opusdei]

Aha, "float" means dividing voltage with the loads. As long as alternator voltage regulator stays below 14.6V, relay charging should be fine.

 

[John Frum]

Aha, "float" means dividing voltage with the loads.

Ideally the charger and the battery share the load.

As long as alternator voltage regulator stays below 14.6V, relay charging should be fine.

Not really.
If you leave the battery exposed to voltage greater than the resting full voltage of the battery it is stressful for the battery.
resting full voltage for a 4s lifepo4 battery is ~13.5 volts.

 

[Opusdei]

Ideally the charger and the battery share the load.

Not really.
If you leave the battery exposed to voltage greater than the resting full voltage of the battery it is stressful for the battery.
resting full voltage for a 4s lifepo4 battery is ~13.5 volts.

 

[Opusdei]

As shown in the LiFePO4 state of charge graph, 3.5V corresponds to 80% SOC. I think it is perfectly normal to use an alternator to charge both type of batteries.

 

[John Frum]

As shown in the LiFePO4 state of charge graph, 3.5V corresponds to 80% SOC. I think it is perfectly normal to use an alternator to charge both type of batteries.

Let us know how it turns out.
Good luck.

 

[Opusdei]

Let us know how it turns out.
Good luck.

According to the Battery University data, 4.2V is cutoff for life cycle reduction due to overcharging. I think there is not much difference in capacity when charged at 3.4V or 4.2 volts as shown in the attached picture. 3.5V seems to be perfectly normal, isn't it?

 

[HaldorEE]

My alternator voltage is around 14V and house battery works fine with relay charging. I am wondering if there is a simple way to limit such overcharging.

What chemistry is your house battery?