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Old 10-12-2017, 12:22 PM   #1
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Word of warning for LiFePO4 battery conversions (Battle Born)

I haven't looked at the voltage profile for other LiFePO4 packs, but I installed Battle Born batteries in my 22FB. So far it was a fantastic upgrade, but there are some trade offs. I will detail the ones regarding converters separately, this is a warning about leaving your TT (travel trailer) connected to the TV (tow vehicle).

Anytime there is a voltage differential between batteries, energy is going to flow from the higher energy (voltage) battery to the lower energy battery. In this case the resting voltage of a 100% charged Battle Born LiFePO4 pack can be as high as 14.4V, over some time it will rest down to 13.6-14V or so (based on observations thus far). 100% charge on your TV lead acid battery is likely 12.7V, and we know from the many discussions here that holding a lead acid battery at high voltage "cooks it". By leaving the TT connected to the TV you are essentially presenting your TV battery with s sustained voltage of up to 14V, while also bleeding capacity out of your TT and largely wasting it as heat.

What alerted me to this is that I would see a 1-2amp steady draw on my house batteries after the TV was connected and the ignition was off. It actually took me a few minutes to decode what was going on, but disconnecting the 7-pin umbilical and the draw stopped immediately. Connect the 7-pin and the draw returned. YMMV may vary, but to protect the lead acid battery in the TV and to maintain SOC on your TT battery you may not want to leave them connected when not actively towing. Eventually my LiFePO4 batteries would have dropped to the same resting voltage as the TV battery, however that is actually a pretty low SOC (state of charge) and not desirable by any means.

As more TT start to use these lithium packs we will all learn more of the nuances of mixing them with our TVs and other devices that are intended for lead acid (e.g. converters). In the ideal world we would have a voltage differential monitor/relay that would manage the connected state of the charge circuit from the 7-pin, it would disconnect it whenever the TT voltage is higher than that of the TV. I am guessing such relays are in common use in RVs to manage state of charging the house batteries, while also isolating the starter batteries and perhaps something could be adapted.

As with all things, YMMV, however I want everyone to go into this eyes open and not be caught by surprise when their boon docking is cut short by unexpected low SOC.
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Old 10-12-2017, 12:33 PM   #2
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Hi

Many TV's have an option for disconnecting the TV power feed to the 7 pin connector. In some cases you pull a fuse. In other cases it's a setting buried somewhere deep in an obscure menu.

Bob
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Old 10-12-2017, 12:48 PM   #3
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Quote:
Originally Posted by uncle_bob View Post
Hi

Many TV's have an option for disconnecting the TV power feed to the 7 pin connector. In some cases you pull a fuse. In other cases it's a setting buried somewhere deep in an obscure menu.

Bob
You may be right, so for those that have the option perhaps it can be configured to only "connect" when the TV engine is running. In my case my Audi doesn't have any configurations for it that I have found, unless perhaps it is not in an authorized menu and is available through my 3rd party "hacking" tool. I will research that a bit more.

Another aspect of this is the risk to your TV battery if left connected while the TT is on shore power and your lithium converter is putting out a steady 14.4-14.6VDC, which would likely be very bad in the long term for the lead acid starter battery.

In looking relay options, it seems that most ACRs (auto-charage-relays) operate on fixed voltage and switch to "combined" charge mode when *either* battery is 13.6V or greater for 30 seconds (based on Blue Sea documentation). This is definitely not the targeted outcome, as the TT battery would always be in excess of 13.6V when at a high SOC.

So far it seems that all of these options look at both batteries as a potential source for charging, Magnum Energy offers the ME-SBC which is adjustable to an extent...so at least you could set the high voltage disconnect to protect the TT battery from over-voltage. It likely won't solve for the other issue as it looks at the voltage of both batteries, and if *either* is above the "connect voltage" then they are connected (which the TT would almost always be). The lowest value on the high voltage disconnect is 14.0V, which is still too high to solve the draining of capacity problem.
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Old 10-12-2017, 01:00 PM   #4
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In my case I think the easiest option is to just install a simple relay between the 12V+ feed within my TV and the 7-pin connector, the relay will be powered by a "switched" circuit (e.g. the rear 12V power receptacle). When the ignition is on and the 12V power receptacle is active, the relay would be closed/connected...and when the ignition is off it would be disconnected. Thankfully I can just do this in my garage and not have to go to the storage facility to work on the AS, so I can do it during the off season in prep for our next trip.
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Old 10-12-2017, 01:21 PM   #5
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Having a relay on the TV that disconnects the umbilical charge lead when the ignition is off is a darn good idea. Might save your cheese in a long campout with the TV hooked up....

Now where did I stash that nice spare relay....
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Old 10-12-2017, 01:40 PM   #6
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Another idea might be placing a Schottky diode in series with the TV charging line. They have a low forward voltage drop and can handle quite a bit of current. They cost about $4 each and don't wear out!
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Old 10-12-2017, 02:55 PM   #7
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Quote:
Originally Posted by rmkrum View Post
Having a relay on the TV that disconnects the umbilical charge lead when the ignition is off is a darn good idea. Might save your cheese in a long campout with the TV hooked up....

Now where did I stash that nice spare relay....
My 2006 F-150 had such a relay built in. One of the "improvements" they have made in the new trucks is to sense that the trailer is connected (lights circuit?) and turn on the charge power. My Ram just has it on all the time, trailer or not.

Al
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Old 10-12-2017, 04:30 PM   #8
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Another idea might be placing a Schottky diode in series with the TV charging line. They have a low forward voltage drop and can handle quite a bit of current. They cost about $4 each and don't wear out!
Those do look pretty perfect, and i doubt the loss of 600mV for charge capability from the TV to the TT would end the world. It looks like my alternator may operate at 14.2V in normal conditions, so that would provide ~13.6V to the TT..which is still within realm of 100% SOC, it would just charge more slowly due to the lower differential. This would likely be a good thing as I can imagine if the SOC in the TT was "too low" and the TV is outputting 14.2V that the battery could definitely draw more amperage than the charge circuit is fused for, assuming voltage drop due to the wiring didn't limit it first.

I like solid state simple things, and it would only take a few minutes to install it within the TT...of course it does mean working within the horribly tiny box that all of this sits within, it takes me days for my neck/arms to recover from contorting to do much in the 12V box
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Old 10-12-2017, 05:23 PM   #9
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Yup, I diode makes sense to me.
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Old 10-12-2017, 07:19 PM   #10
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The forward drop is 600 mV at high current, as the current drops off, so does the forward voltage. You could also mount the diode in the trailer if thatís easier.
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Old 10-12-2017, 07:20 PM   #11
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You need to install a Victron Orion 12/12 DC-to-DC converter to correct this issue and convert the inbound 13.X volts from the TV of the correct 14.X voltage in the trailer. The Orion converters are designed for this very purpose: voltage normalization across disparate systems. Once installed and configured, your TV will actually charge your Lithium at the correct voltage, vs. draw from it.
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Old 10-12-2017, 11:06 PM   #12
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Originally Posted by alano View Post
Another idea might be placing a Schottky diode in series with the TV charging line. They have a low forward voltage drop and can handle quite a bit of current. They cost about $4 each and don't wear out!
yes

i totally agree with this simple automatic approach.
noting to think about
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Old 10-13-2017, 10:17 AM   #13
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Quote:
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Another idea might be placing a Schottky diode in series with the TV charging line. They have a low forward voltage drop and can handle quite a bit of current. They cost about $4 each and don't wear out!
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?

Thanks.
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Old 10-13-2017, 10:19 AM   #14
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Hi

If you take a look at many automotive charging systems, they put a lot of voltage on the battery on a pretty regular basis. Finding one that idles at > 14V is not uncommon. Finding one that runs below 13.4V is rare. Most lead acid battery spec sheets call out a long term "float" voltage in the 13.2 to 13.4V range (at 70F) as an acceptable thing.

If you go with a diode, accept that a "house battery low" condition can occur. In that case, whatever your alternator can push through the wires will go through the diode. You *can* get very high current diodes. Make sure that you follow the heatsink rules on them. They will melt internally if you don't .... I have data on this ....

Bob
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