backchannel: RebelBbatteries FAQ YT video

Full disclosure: I like the guy, I like the company, and I love my Rebel Batteries LFP. I have mentioned before that I think Aric shoots himself in the foot when discussing/comparing LiFePO4 to lead (Li and Pb hereafter).

the video

Top 6 LiFePO4 Battery Questions from Real Customers

The timestamps I use below might be slightly off because of the offline processing I do to youtube videos.

What kind of charger do you recommend? (0:22)

… lithium, which is obviously the preferred type of charger. (0:30)

I would much prefer a configurable Pb charger to a nonconfigurable Li charger.

If you understand the voltages… it will make a little more sense about why each works or doesn’t work with lithium batteries.

Indeed.

A typical lead-acid battery will charge in the high-thirteen range. It might go above that but it will drop back down pretty quickly.

No. FLA will charge (absorp) in the mid-to-high fourteens and hold that voltage for several hours (absorption duration). After that it will float in the mid-thirteens.

most people will say 14.2v to 14.6v for LiFePO4 (1:17)

Most people also think it’s Berenstein bears but that doesn’t make it so.

Given that 3.65vpc is the limit the BMS will shut off charging to prevent cell damage 14.6v (3.65 x 4 cells) seems needlessly high.

I have taken some guesses about why recommended charging voltages are so high in this RVwiki article. The TL;DR is that battery sellers are interested in minimizing support contacts and making it through the warranty period.

… so that’s kinda makes sense why a lead-acid charger wouldn’t work very well with a lithium battery (1:22)

Because Pb chargers don’t Absorp at 14.2v-14.6v? Let’s look at some popular examples and their Pb-charging default settings.¹

charger	FLA	AGM	Gel
Renogy	14.6v	14.4v	14.2v
Victron	14.4v	14.4v	14.4v
EpEver	14.6v	14.4v	14.2v

I didn’t cherry-pick these; I googled manuals for the first three brands that came to mind. This is knowable stuff.

Foreshadowing: none of these have a Vbulk setpoint.

AGM is a type of Pb battery but it takes a higher voltage (1:30)

No. Vabs for AGM is typically lower that for FLA due to the risk of permanent damage from outgassing. Hence the patterns in the defaults seen above.

There’s a caveat to AGM {charging} though because sometimes AGM {charging profiles} will have time limits. Like they’ll have a max 4-hour charge cycle then they may just stop… (1:45)

That is Absorption duration. Li charged at ≥14v and moderate currents likely needs no absorption at all.² Charging 13.6v - 13.9v will likely require some absorption duration.

lead-acid {chargers} don’t even try, it’s a waste of time. You’re not going to get more than 20-30% on the charge. It’s going to take days to get there. (2:25)

My fully-charged Rebel 100Ah begs to differ. 99% SoC after charging to 13.8v with 20 mins of duration, and dropping to 13.4v float. It took ~3 hrs³ to go from 50% DoD to 100% DoD this morning.

How can I charge from my alternator? (2:38)

The alternator will take all the amps the alternator can send. (3:10)

My Rebel LFP draws ~30A from the 180A alternator using an isolator. Works great.

What settings should I use for Bulk, Absorption, and Float (4:15)

Aric conflates the stages here. There are typically no Bulk voltage settings as it is a Constant Current stage.⁴

Correctly stated, Absorption voltage according to him should be ~14.4v.⁵ and Float at ~13.6v.⁶. His discussion of current’s affects on SoC at terminal voltage (5:00) undercuts the 14.4v setpoint logic and (unintentionally) makes the case for 13.8Vabs.

Why did I lose X percentage (of SoC) overnight? (6:08)

The real answer: we don’t know how the underlying JBD BMS estimates SoC and we shouldn’t worry about it.

It appears

1. the BMS estimates SoC by counting amps, not by voltage
2. the BMS resets the counter after passing a voltage setpoint⁷

If the user is experiencing actual capacity loss overnight I’d be looking at parasitic draw, including the internal BT dongle.

Note: I use an external shunt on the battery and it tracks within 1-2% of the BMS’ estimate. My shunt is set to 100Ah and reset to full at 13.8v.

The most interesting part of this section is that RB underrates the 100Ah; it is actually 104Ah. (11:05) This is A Good Thing, allowing a bit more leeway at very low SoC.

What do I do with a battery in cold weather? (15:15)

This section was fine.

How to store a lithium battery? (17:00)

…leave them a about 80% {state of charge} and totally disconnect them…

Sure. 80% might be high (I’d shoot for 40-50%) but it’s better than fully charged. He later mentions 75% as another storage point.

It would have been smart here to mention disabling the bluetooth dongle on models so equipped.

bonus question - Do I need a separate battery for my electronics (19:14)

I have no expertise on this part.

comments

twitter comment thread for this post

1. All of these are broadly user-configurable. ↩

2. aka “charge and stop”. Note that Renogy Li chargers have no float setting at all. ↩

3. not days ↩

4. some high-end controllers have a separate BulkV but this is rare. ↩

5. and 13.8-14.0v according to me ↩

6. I’d recommend 13.4v to hold the voltage rather than continously charging ↩

7. I would refer to the RB spec sheet for the BMS but it is 404 and wasn’t captured by the archive.org spider. ↩