Batteries: AGM vs LiFePO4

[BrucePerens]

I have the Victron BP-100 battery protector. The connector for the BMS is just a switch, there is no data communication. You can wire a plain switch to it, and it will turn power on and off. Apparently there are some cases in which the BMS wants to turn off the output power. But the protector definitely works standalone. The battery protector itself is programmable for the shutdown voltage you want. I got mine from pkys.com . The BP-65 is sufficient.

[rickst29]

I looked at the BP-65/100/220 manual, and saw the following table of choices for disconnect/reconnect voltage values. As you see (from the jpg), you can't program your own values - you must choose form the list, or choose "Lithium Mode" (which puts in entirely under the control of a BMS).

For everyone (except Bruce, who already knows this): Discharge of a "12.8V" 4-cell LFP battery, at low current (<30A) doesn't present a smooth drop-off of Voltage, in the way that Lead-Acid batteries do. Instead, it's nearly like falling off of a cliff - Voltage stays nearly steady, until the battery is 95% discharged, and then falls very rapidly. For a TM, the most desirable low-voltage cutoff value is probably between 12.0V and 12.4, with "reconnect" at anything above about 12.8V.

Victron programming does not provide for programming at values which are this close together. The highest "cutoff" Voltage choice, 12.0V, is matched to a "reconnect Voltage of 13.0V. It's a bit lower than I like but it is the best choice available, and I'm going to get one when I proceed: It's MUCH higher than the built-in BMS cutoff, and can increase the lifespan of the battery by preventing a lot more "over-deep discharge" situations.

[BrucePerens]

You're welcome. It's just so darn easy to leave things on, and even losing a pair of lead acid batteries is annoying. I don't recommend that anyone be without a battery protector.

I don't see anything that can come closer to your limits other than unencapsulated Chinese boards on eBay that I'm not ready to trust in this application.

[rickst29]

Originally Posted by BrucePerens

The old solar controller will still work as a battery monitor if you disconnect the solar panels from it. Don't throw it out.

Modern MPPT chargers are cheap. Figuring out where to add panels and how to connect them will take some calculation. Where is the present one mounted?

Hmmm - I recommend that you do throw it out. It has too much power draw, and you can great a GREAT battery monitor which measures and Displays both Voltage and power-in power-out (Ah) as a "coulomb accumulator". It also shows instantaneous current. Costs about $40 with shunt included from Alibaba. On Amazon, an equivalent costs about $60 and does not include the necessary shunt.

This one works for LiFePo4, as well as Lead-Acid battery types: https://www.aliexpress.com/item/8-80...885265096.html. A simple $5 "Voltage Only" Battery Monitor will work fine for Lead-Acid, but utterly can't work for LiFePO4 (because the output Voltage of an LFP battery is nearly constant from 90% State-Of-Charge all the way down to less than 20% State of Charge).

[rickst29]

About an hour ago, I plugged in my 150ah LFP battery (somewhere in the range of about 40%-60% SOC) into my new PD Converter ("Wildkat 4655L").

The resting battery, with no load except for the tiny current to run the voltmeters, showed 13.3V (according to two very good meters, which did agree with each other). Using the Charge Wizard, I invoked 14.4V "Boost" mode. With barely 1.1V voltage differential, the battery started to absorb everything which the "WildKat" was willing to push out (all 55 amps).

Now (after roughly 55ah more into the battery), voltage on the charging battery terminals has increased to 13.95V. That's less than 1/2 Volt differential. But the battery is still accepting over 53 amps! Conclusions:

1. If you replace lead-acid batteries with LFP, they can be re-charged REALLY FAST.
2. If your Converter can't throttle it's output, or can't prevent itself from overheating - it will probably DIE after you install a battery like this one.

I'm amazed. I've never seen this kind of behavior from an SLA battery. When you upgrade to LiFePO4, you must increase the wire sizes (and fuse/Circuit Breaker) between the Battery and the Converter, to support the entire output of the Converter. The battery creates hardly any "internal resistance" during charging.

[BrucePerens]

How hot does the battery get while accepting a high current charge?

[rickst29]

Originally Posted by BrucePerens

How hot does the battery get while accepting a high current charge?

My new battery is built within a plastic case, which might some some insulation value. All my testing has occurred on the living room floor, room temperature 71F. During the charge cycle (CC @64A followed by CV @ 14.4V with declining amounts of current), the upper case temperature was slightly higher than room temperature - quite hard to detect, maybe 1 degree C. The lower case temperature was identical to the room.

It felt as if the battery temp was totally unaffected, with just a tiny bit of heat emanating from the BMS circuit board. This was a strange experience for me- with previous SLA batteries, I've always felt increasing temperatures (sometimes quite large).

[rickst29]

except for a 30A Breaker on the "tiny" (#10) red wire, which goes to a new exterior 12V port. (That Breaker is on order, and should arrive shortly.)
Photo: Battery-Connections.jpg

Complete Discussion: LiFePO4 Upgrade In Progress.

I like it, and would not dream of going back to lead-acid.