Word of warning for LiFePO4 battery conversions (Battle Born)

[guskmg]

When "lewster" did my solar/lithium system we simply pulled the truck's 50 amp fuze to isolate the TV/TT aux power system. There is plenty of on board battery power for the brakes. Another cheap option is to disconnect the TV/TT incoming power at the umbilical J box inside the left front corner of the trailer. That maybe the better option as you may pull the TT with a different TV. WARNING: connecting lithiums to an uncontrolled source of charging is an invitation to disaster. The best thing to do is to ask the manufacture of the lithiums.
guskmg

[SilverHouseDreams]

Originally Posted by guskmg

WARNING: connecting lithiums to an uncontrolled source of charging is an invitation to disaster. The best thing to do is to ask the manufacture of the lithiums.
guskmg

You are correct, in the case of Battle Born the battery management is internal to the battery itself. That includes temperature control, over voltage and under voltage cutoff and cell balancing...that is what makes these "drop in replacements", however I am trying to warn people of the unintended side effects that they could experience. So rather than buying a $200+ voltage regulator (Victron Orion) I would rather just bypass the problem.

The Blue Sea devices mentioned likely wouldn't work all that well in this case, as they seem to happily detect a "charge" on either battery based on voltage. By my understanding they would likely always detect the higher voltage of a lithium pack as having an active charge source and would then connect the two sides.

I'm not as concerned about the TV battery, as noted my particular TV likely "floats" at 14.2V and likely has a much higher recovery/charge voltage. The bigger problem is that you might expect to start your journey with a full house battery, which may not actually be the case if you had it tethered to your TV in a powered off state for "long". I didn't sit around waiting to see how long the 2-amp draw would last, or if the draw would decrease over time. It is possible that my TV would have cut the feed at some point, as it does have a timeout that turns off almost every other "accessory" that could drain the starter battery.

I do like the potential to have the house battery charge while on a long drive (even a 10amp charge rate with a 4 hour drive is ~40% capacity recovery)...which for a multi-stop boon docking trip would allow us to entirely avoid using a generator (which we don't currently own).

[zapper]

Although not expressly mentioned, but inferred in the comments above, diodes have a polarity and must be installed in line with the + side in the direction of current flow from the TV to the TT. To do otherwise gives an open circuit indication and nada happens.

[uncle_bob]

Hi

If we are getting into all the issues on diodes, one very common issue: The diode protected my fuse ...

Diodes blow pretty fast when hit with over current. They blow faster than conventional fuses. You *can* get very fancy fuses to protect them. Even then, there is a bit of derating of the diode involved.

For the same money as a cheap diode and very expensive fuse, you probably can put in a MOSFET module that is quite a bit more robust. It also will have less voltage drop than the diode.


Bob

[rmkrum]

As a long term electronic fiddler, engineer, and ham radio guy, I’ll stick with a nice mechanical relay that only turns on the charge line to the AS when the TV ignition is on. Simple, easy and reliable.

[alano]

As a long term electronic fiddler, engineer, and ham radio guy, I’ll stick with a nice mechanical relay that only turns on the charge line to the AS when the TV ignition is on. Simple, easy and reliable.

As a long-term electronic fiddler, engineer and ham radio guy myself, I would respectfully disagree. I'll take a solid-state solution over a mechanical relay any day!

Two questions;
1. Only applicable if you have the new whiz bang batteries?
2. Can provide a bit more detail for us old mechanics with only slightly noticeable electrical burn scars?

1. A diode will prevent current to flow from the trailer to the TV with any type of batteries.
2. A diode has two terminals: an anode and a cathode. Current will only flow in the direction from anode to cathode. So in this example, the anode would connect to the +12V line from the TV, and the cathode would connect to the TT. If the voltage on the TT is ever greater than the TV, no current would flow.

The real gotcha is the heat involved. If you can have 40A down the 7 pin and 0.6V on your diode, you have 24W into the heatsink. In most cases, you would do that with either a big heatsink in free air or with a fan on something smaller. Packing this away behind a bulkhead is not going to be "free air" in terms of convection.

The diode should be connected to a heat-sink. At 40 A the diode I specified will dissipate about 20 W. One of the nice things about Schottky diodes is that the forward voltage decreases with temperature, so as the diode heats up, the forward voltage will drop and the power dissipation will decrease. A 20 W heat sink is fairly small - about 3" x 2" by 1". Bolting the diode to a steel frame with heat sink grease would work extremely well. Since the backside tab of the diode connects to the cathode, an insulator is required to make sure the cathode tab remains isolated. These insulators are also sold at Digikey.

Diodes blow pretty fast when hit with over current. They blow faster than conventional fuses. You *can* get very fancy fuses to protect them. Even then, there is a bit of derating of the diode involved.

In this case the TV likely has a 40 A fuse in series with the +12 V line. The diode I selected can handle 60 A continuously and a single pulse up to 600 A, so I think it's unlikely that the diode would fail before the fuse.

For the same money as a cheap diode and very expensive fuse, you probably can put in a MOSFET module that is quite a bit more robust. It also will have less voltage drop than the diode.

A P-channel MOSFET could work, but only as a replacement to a relay. It would also require an additional wire and switch and require user intervention. It could be made automatic, but that would require additional circuitry which would certainly make it too complex for most folks and cost substantially more than a diode. Even so, a P-channel MOSFET has a back-diode, which means that when the MOSFET is off, current can still flow from the TV to the TT if the trailer's +12 V is a couple of volts less than TV. In this case the same situation could occur where the TV can supply 40 A (before the fuse would blow) and the MOSFET could overheat. In this case the MOSFET would need a substantially larger heatsink since the forward drop of the back diode is typically 3-5x of a Schottky diode!

Anyway, it's always fun nitpicking over these corner cases. After reviewing the comments, if I had a lithium battery installation I would probably consider the simply diode solution IMHO.

[uncle_bob]

Originally Posted by alano

......




In this case the TV likely has a 40 A fuse in series with the +12 V line. The diode I selected can handle 60 A continuously and a single pulse up to 600 A, so I think it's unlikely that the diode would fail before the fuse.



....

Hi

If the 40A fuse is a standard automotive part, it will pass *way* more than the 600A the diode is rated for (in the time period the diode cares about). The bigger question becomes - will the wire resistance allow more than 600A? Maybe it will maybe it won't. If it won't that's not always a good thing. 400A for 4X the time probably still blows the diode. The fuse likely slows down with lower peak currents.

I've replaced a *lot* of diodes over the years because of these sort of problems ....

Bob

[wulfraat]

I am a little baffled at why one wouldn’t correctly normalize the inbound TV voltage using a DC to DC converter if you want these 2 disparate systems to co-exist for house battery charging purposes? Is cost the issue for a $200 converter? Welcome to the world of Lithium. Nothing is ever ‘plug and play’

If one wants to disconnect the TV voltage altogether on the cheap then just remove the 10ga 12v+ wire inbound from 7-way wiring harness from the positive bus bar in the trailer and be done with it cost is $0 and takes 5 minutes. Trailer brakes will still work fine as there are controlled by a separate wire on the 7-way within an isolated / closed circuit.

[interstateflyer]

Does anyone happen to know if Dodge incorporated safeguards against AS to TV flow in the 2016 RAM 2500?

[uncle_bob]

Originally Posted by wulfraat

I am a little baffled at why one wouldn’t correctly normalize the inbound TV voltage using a DC to DC converter if you want these 2 disparate systems to co-exist for house battery charging purposes? Is cost the issue for a $200 converter? Welcome to the world of Lithium. Nothing is ever ‘plug and play’ .......

.

Hi

The "cute" way to do this is to feed the DC/DC into the solar side of things. That way you get a proper charge controller "for free". You might need voting diodes, so not quite free.

Indeed there isn't a lot of charge available from the TV. It *is* enough to get the trailer battery up to 11V from "dead" fairly quickly. This matters if you arrive at storage to find a dead trailer (and want the break away to work). I have data on this ....

Bob

[wulfraat]

Originally Posted by uncle_bob

Hi



The "cute" way to do this is to feed the DC/DC into the solar side of things. That way you get a proper charge controller "for free". You might need voting diodes, so not quite free.



Indeed there isn't a lot of charge available from the TV. It *is* enough to get the trailer battery up to 11V from "dead" fairly quickly. This matters if you arrive at storage to find a dead trailer (and want the break away to work). I have data on this ....



Bob

If you have an mppt solar controller (victron anyways in my experience) this won’t work because the controller needs to see a 5v delta between source and destination in order to activate current flow - so charging would never be activated unless you are feeding it >18v open circuit voltage on the source side. TV won’t generate that level of voltage, only a solar panel will in a 12v system.

I stand by my statement that the correct way to address this is a DC-DC converter specifically designed to normalize voltage across the 2 systems (house vs TV 12v platforms)

[SilverHouseDreams]

I think the key problem with the Victron DC-DC converter is that it is only 120W, that seems a bit limited for $200+. I will likely just go with the relay option within the TV, as it just seems like the "right way" to go about solving it for my issues. The diode is also an appealing option, though it is definitely more complicated and outside of my area of expertise (planning heat sync, electrically insulated thermal transfer, etc).

[wulfraat]

The 12/12 has 110/220/330 watt options.

Agree though, that disconnecting the 12v TV lead is the simplest approach if you do not need a charge from the TV while on the road.

[gklott]

Originally Posted by wulfraat

The 12/12 has 110/220/330 watt options.

The Victron Energy Orion 12/12-30A (360W) Isolated DC-DC converter p/n ORI121236100 is a 9-18 VDC in, 12.5 VDC out, 360W converter. The output appears to be fixed at 12.5 V, and the manual on line does not offer any output V adjustment.

The Victron Energy Orion-TR 12/12-18A (220W) Isolated DC-DC converter p/n ORI121222110 has 8-17 VDC in, and adjustable 10-15 VDC output.

Are these the ones you are considering?

73/gus

[wulfraat]

You can adjust output voltage on all 3 models.

https://www.victronenergy.com/upload...50-400W-EN.pdf

Not that it matters as you’d have to bypass the 7-way plug and run a new heavier gauge dedicated charge line from TV to trailer for anything over 10A.....

[james.mileur]

Run a new wire? DC - DC: how do you get more than 10 amps out if you don't have 10 amps coming in? You would have to drop voltage down below charging level to raise amps?

[wulfraat]

You could pull a 8ga or 6ga 12v lead right off the alternator of the TV I guess to deliver >10A inbound - just guessing. Never contemplated it until this thread

[larry9000]

I measured the voltage at the 7 way plug on my 4runner. It's keyed to the ignition.

I get somewhere around 12.2V if the key is on, but engine not running.I get 13.9 when the engine is running. The voltages match the battery voltages, so there's no drop to the plug. Voltage goes to zero when key is out.

So, I don't see a problem if the plug is keyed to the ignition. The lithium won't charge the battery when TV is parked, as the ignition will be off. When turned on, but engine not running, is the only time there would be reverse current flow. But that only lasts for seconds before starting the TV. Voltage goes to 13.9 when alternator is running. I don't think that will deplete the lithium battery or the TV battery as the alternator is putting out good current. Am I missing something?

[uncle_bob]

Originally Posted by larry9000

I measured the voltage at the 7 way plug on my 4runner. It's keyed to the ignition.

I get somewhere around 12.2V if the key is on, but engine not running.I get 13.9 when the engine is running. The voltages match the battery voltages, so there's no drop to the plug. Voltage goes to zero when key is out.

So, I don't see a problem if the plug is keyed to the ignition. The lithium won't charge the battery when TV is parked, as the ignition will be off. When turned on, but engine not running, is the only time there would be reverse current flow. But that only lasts for seconds before starting the TV. Voltage goes to 13.9 when alternator is running. I don't think that will deplete the lithium battery or the TV battery as the alternator is putting out good current. Am I missing something?

Hi

Well, first off, 12.2V is not a very encouraging battery voltage. On either the trailer or the TV with lead acid it should be up around 12.7 with the engine off and nothing going on.

The drop in the connector and wiring is proportional to the current flowing. With no current, there will be no drop. If you have (say) a one ohm resistance in the wire, you will get a 1 volt drop for every amp of current that flows. Obviously a real wire isn't going to be one ohm.

In the case that your TV does not switch off the 7 pin with the ignition off (some have a delay that is measured in hours), you will get a back feed to the TV battery. How much current depends on the wiring resistance.

If your Lithiums are sitting there putting out 14.5V and your alternator only gets up to 13.9, the Lithiums will try to run the TV through the 7 pin cable. To the degree they succeed at doing this, you will run down the Lithiums.

Lots of combinations and permutations ....

Bob

[DannyClark]

Good Grief. I’ve got two Battleborns inbound. Is nothing “easy” lol?