Going Lithium questions

RACarvalho · 01-26-2022, 02:18 PM

Ok, so i was planning to upgrade my batteries to AGMs and now with the price of lithiums going down it will make more sense to go for them so my question is:

BIM:
The BIM I have today is 160A but Precision Circuits is only offering the 225A version for Lithium....
As far as I know, the engine alternator is 240A so my question is:
If the BIM allows 225A to go to house batteries, cant this overload the alternator since there are other loads drawing from it too?
What is the cable size for a 225A load?

Converter:
I have the 55A version and WFCO have a 75A lithium ready version.
What size of cable is required for the 75A?

Thank you for any input!

DavidEM · 01-26-2022, 07:52 PM

Your lithium batteries would have to be nearly discharged to draw 225 amps and it would take more than 200 amp hours of capacity as most lithium battery's BMS limits charging to 100 amps.

Also the wire size plays an important part in the maximum current drawn from the alternator. If you wired it with 10' of #2 wire which is a typical size but isn't quite big enough to safely carry 225 amps (210 actually) the voltage drop would be 0.7 volts. If the alternator is putting out 14 volts that means 13.3 volts is going to the batteries which is probably too low to charge at 225 amps. Also I doubt if the alternator would put out 14 volts if you tried to draw 225 amps from it.

So the first thing you can do is replace the BIM 160 with a Li BIM 225. This BIM charges for 15 minutes and rests for 20 and then 15 again...... That will reduce the average load on the alternator.

The best solution is to install a DC to DC charger. That will both limit the current to the rating on the charger (varies from 10-60 amps depending on brand and model) and boosts the charging voltage so the Li batteries get fully charged.

For your 75 amp Li converter/charger if it is within 10' of the battery, #2 will result in a maximum voltage drop of 1/4 volt which is what you want for best charging performance.

David

RACarvalho · 01-26-2022, 08:01 PM

Quote:

Originally Posted by DavidEM

Your lithium batteries would have to be nearly discharged to draw 225 amps ...

Thank you for this information David.
I found out that the battery BMS have a limit at 100Amps...
The House batteries are 12ft from the converter in a straight line and 14ft from the alternator.
What DC to DC you recommend and where to locate it in the circuit?

Tks,
R

DavidEM · 01-26-2022, 09:04 PM

If the converter is a 12' straight line from the batteries then it is probably at least 15' overall. So I would wire with #1 gauge.

I think that most MH alternators can take a continuous load of 60 amps while charging the house batteries. Renogy makes a 60 amp DC to DC charger. $176 on Amazon.

Wiring is a bit more problematic. A DC to DC charger is unidirectional so it is incompatible with your BIM. So remove the BIM and wire the DC to DC charger in its place. You will use the chassis battery connection, the coach battery connection and the ignition wire to hook it up. That leaves the Sig wire. You can install a latching relay in parallel with the DC to DC charger and use that Sig wire to close the relay. That will let you start the chassis engine from the coach batteries as in using the aux start function. Or just wire in a simple heavy duty on off switch to parallel the two DC systems when necessary. That will also let the coach batteries charge the chassis batteries while in storage.

David

RACarvalho · 01-27-2022, 01:16 PM

Quote:

Originally Posted by DavidEM

I think that most MH alternators can take a continuous load of 60 amps while charging the house batteries. Renogy makes a 60 amp DC to DC charger. $176 on Amazon....

Again, Thank you.

Do you have the latching relay specification / model you would use?

Also, in your suggested set up, If I have a problem that leads to drain my house battery completely, will that drain the chassis battery too?

Sorry, my understanding of auto electricity is limited.

Tks,
R

DavidEM · 01-27-2022, 03:22 PM

No I don't have a specification or model number of a suitable latching relay. Actually I think a simple switch is better for the once in a lifetime chance that you need your coach batteries to help the chassis batteries start the engine, or keep the chassis batteries charged while in storage with shorepower keeping the coach batteries up. Go on Amazon and look for switches capable of handling 250 amps DC. I would prefer a rotary one.

With no switch or relay on, just the DC to DC charger in place of the BIM, there isn't any way of running down the chassis batteries. The DC to DC charger acts like a diode and eliminates any current flow in that direction.

David

RACarvalho · 01-28-2022, 01:36 PM

Thank you David!
I was wondering what happens if I just add the DC/DC as per the picture below?
If I'm understanding right, when the engine is on, the DCDC charger will be charging the house batteries so the BIM will not close and connect the two batteries.
Then when at shore power, the DCDC charger is off and if the chassis battery voltage goes down, the BIM connects and allow the house converter to charge the chassis battery...
in this case I would:
1 - Not have to change the BIM since the "responsible" for charging the house batteries would be always the DCDC charger
2 - Keep the "emergency start" function the BIM has as it is today.

Am I right?

DavidEM · 01-28-2022, 02:17 PM

Looks like you are proposing to wire the DC to DC charger in parallel with the BIM. Doing that will allow the DC to DC charger to supply charging current and the BIM to also do that without limit, so the current could be excessive.

Also the DC to DC charger if installed normally creates a low impedance load on the chassis batteries that allows it to supply its rated current to the batteries even if the chassis alternator's voltage is too low to do so directly. With the BIM in parallel, I don't think this will work.

Better to replace the BIM completely with the DC to DC charger, with or without a bypass switch or relay.

FWIW, this discussion is getting a bit too theoretical for me. I suspect that neither you nor I have enough experience to get this right by ourselves. I would communicate with the DC to DC charger manufacturer to get their ideas. This has certainly been done before and they should be able to help you with the solution.

Renogy, Victron, Redarc, all have good customer support people who can help. If you do get meaningful help from them, post the results here. I have yet to see an RV owner install a DC to DC charger with charging of the chassis batteries or aux start capability and report on it in this forum or a few others I follow. The only report I can remember was a simple swap out of the BIM with the DC to DC charger.

David

RACarvalho · 02-01-2022, 04:57 AM

Hi David,
The idea of a DC-DC charger is growing in my mind...
Would this relay work for the emergency by pass?
https://www.amazon.com/dp/B08F28SKX1...mrai_1_dp?th=1

Thank you!
R

DavidEM · 02-01-2022, 12:29 PM

It should work.

David

rh70 · 02-01-2022, 08:31 PM

I would love to switch to lithium, but I'm to stupid. You guys lost me at the bakery.

Chance · 02-01-2022, 10:17 PM

Quote:

Originally Posted by DavidEM

No I don't have a specification or model number of a suitable latching relay. Actually I think a simple switch is better for the once in a lifetime chance that you need your coach batteries to help the chassis batteries start the engine, or keep the chassis batteries charged while in storage with shorepower keeping the coach batteries up. Go on Amazon and look for switches capable of handling 250 amps DC. I would prefer a rotary one.

With no switch or relay on, just the DC to DC charger in place of the BIM, there isn't any way of running down the chassis batteries. The DC to DC charger acts like a diode and eliminates any current flow in that direction.

David

David, if using system you describe, I would personally not leave a lithium-specific converter that operates at slightly higher voltage connected to chassis battery for long periods during storage. I would be concerned about overcharging the chassis battery. It may not hurt the starting battery but I wouldn’t take the risk.

There are simple ways around that concern like a separate chassis trickle charger if it was necessary.

RACarvalho · 02-02-2022, 04:14 PM

Quote:

Originally Posted by DavidEM

It should work.

David

Thank you.
I'm designing the system diagram using bus bars and I came to this:
In case the chassis battery is "dead" and we use the switch/relay in order to start the MH, wouldn't we be "shorting" the DCDC charger since we will be joining the input and output on the positive side?
My point is: The DCDC charger will see it has ignition signal so it would want to start charging the house battery but the voltage on the input (chassis side) is too low and then the relay closes and now the voltage in the input is equal to the one on the output BUT below the max for Lithium ... what happens?

RACarvalho · 02-02-2022, 04:50 PM

Quote:

Originally Posted by DavidEM

It should work.

David

I guess I found the right way.
I should buy a 5 pin relay, connect the chassis battery to pin 87, the Charger to Pin 87A and the house to pin 30.
In this way, chassis is always connected to the charger since 87 to 87A is a NC but house is always disconnected since 87 to 30 is NO.
When the emergency switch is activated the situation is inverted and the relay disconnects the charger and connects the two batteries.... am I right?

Chance,
If this is right then the DCDC converter will be linked to the battery only when the ignition is on...

RACarvalho · 02-02-2022, 04:52 PM

Sorry for the rough drawing, I didn't have to much time to refine it... Just trying to understand the system before I start working on my battery bay layout...

**16ACE27** · 02-02-2022, 06:53 PM

Quote:

Originally Posted by RACarvalho

I guess I found the right way.
I should buy a 5 pin relay, connect the chassis battery to pin 87, the Charger to Pin 87A and the house to pin 30.
In this way, chassis is always connected to the charger since 87 to 87A is a NC but house is always disconnected since 87 to 30 is NO.
When the emergency switch is activated the situation is inverted and the relay disconnects the charger and connects the two batteries.... am I right?

Chance,
If this is right then the DCDC converter will be linked to the battery only when the ignition is on...

Have you found a relay that will handle that current load? Most of those 5 pin relays are rated at 30 amp non-continuous duty although I have seen them as high as 80 amp.

RACarvalho · 02-02-2022, 07:29 PM

Quote:

Originally Posted by 16ACE27

Have you found a relay that will handle that current load? Most of those 5 pin relays are rated at 30 amp non-continuous duty although I have seen them as high as 80 amp.

The DCDC charger side is limited to 60A by the charger.
I understand the 250A is for the momentary emergency start... I found this one:
https://www.amazon.com/dp/B08F2G1J9D...lig_dp_it&th=1

ScottyB410 · 02-05-2022, 11:52 PM

Would this be a better solution? Or am I missing the concept. I am using this with my new install of the battleborn.

BASSMAN · 02-06-2022, 12:47 AM

I am using the Renogy 60 amp DC to DC charger and put it in over a year ago, I put in 4 Gel cell batteries and they require settings like the Lithium do for charging so I disconnected the charging link to the house batteries and use the DC to DC charger as it has setting for the AGM, GEL and Lithium batteries, I do have a mechanical switch to tie house and engine battery in the event of dead battery on either end, do your homework on the Lithium before you buy, lower price is usually lower quality, I have 2, 12 volt 100ah in my boat in series for 24 volt to run a trolling motor, I bought the Dakota as they offered a 24volt trolling motor package and they were $1,700.00 with a 11 year warranty, Lithium batteries need a good battery monitor to know where your batteries are as far as charge, I bought a Victron Monitor that is programable for lithium and has a blue tooth for an smart phone app, I had to buy a marine grade 24 volt charger from Victron thats a 12 amp output, they recommend buying a battery balancer when ever using batteries in series or parallel, all of this applies to motorhomes when using the newer batteries, also the DC to DC charger is powered by the engine battery an not the alternator to charge the house batteries, some of the install information on the DC to DC charger are in error, I had to call Renogy to verify their install information as being in error, my guess is they are still sending out the same wrong information with the unit

RACarvalho · 02-06-2022, 05:44 AM

Quote:

Originally Posted by ScottyB410

Would this be a better solution? Or am I missing the concept. I am using this with my new install of the battleborn.

Scott,
I already have this on my MH but on the regular version. If I install the Lithium I'll have to update it to the Lithium version which is the one you have in the picture.
The problem with it is that it only connects the battery but doesn't regulate current which means whatever the Lithium batteries would accept the alternator will be required to deliver and depending on the situation you can burn your alternator.
The discussion is to get rid of this and install a DCtoDC charger which limits the charge current so there is no risk for the alternator.
The problem with this solution is that it eliminates the "emergency start" feature that the MH have.
To keep that, a relay is needed ...

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