Lead acid or LifPo decision time

[Hedges]

Were you recommending this circuit for output stage of power inverters?
It will require Vbe to reach about 0.7V before operating; that is 0.7V across Rcl.
Consider a 5000W inverter with 50V from battery; 100A flows, so 70W dissipated in 7 milliohm Rcl (or multiple parallel circuits.)
That would be 1.4% of output wasted just in sense resistor. Around one quarter of entire losses of inverter.

A system might have a "50 mV" shunt. If that was a 100A shunt, would be 0.5 milliohm (but more likely to use 400A or so shunt.)



I would think a much smaller value Rcl should be used, with op-amp or comparator to boost the voltage.
I've seen a similar circuit used for a linear current regulator. In that case, for 2A it has 0.25 ohm 4-wire "kelvin" sense resistor. (efficiency was not a concern, rather accuracy and stability.)

Here's an 0.002 ohm resistor, good for 1.5W (27A) without heatsink. 10x the power with heat sink. One of these per power transistor could work.

https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/PWR220S_221S.pdf

Really good point, though, about running the inverter hot and then shorting output to test robustness.
I think much testing occurs under nominal conditions, but what you recommend is better to determine if it will enter runaway and kill itself.

 

[Don B. Cilly]

I recently assembled an LFP battery.
8 280Ah cells gave me... 280x24, 6.72 kWh. As they are over 26V most of the time, (>)7.28.
Total cost, with BMS and shipping, €1200.
Add some wires and a fuse, (which I had) not-much-more.
$7000 seems a lot.
-

 

[scrubjaysnest]

In looking at AGM VS LIFEPO4 it comes down to a different cost area for us, existing PWM or shell out $800 for a LIFEPO4 compatible MPPT on top of the battery cost difference. AZ desert We'll stick with the PWM plus AGM.

 

[Supervstech]

In looking at AGM VS LIFEPO4 it comes down to a different cost area for us, existing PWM or shell out $800 for a LIFEPO4 compatible MPPT on top of the battery cost difference. AZ desert We'll stick with the PWM plus AGM.

Mppt is only needed if you have a large array needing high panel string voltage.
Pwm is plenty useful for lifepo4

 

[krby]

Yet another "expert" chiming-in. IMO the SLA vs LFP decision is situation and user specific. Some things I've considered in past projects:
* Does weight matter a lot (will you be moving the batteries around)?
* SLAs are going to be one of the heaviest options. Will I be cycling the batteries (typical of solar in a house you're living in) or leaving them at 100% most of the time (typical of a backup or UPS-type situation)? Lithium chemistries don't "like" to be at 100% all the time.
* Does self discharge matter? If I'm leaving these without charge for long periods, I need to think about this.
* How cost sensitive am I right now?
* Do I have gear already for the project that can deal with the LFE voltage range and charge profile? It's not the same as SLA.


There are other considerations, but these above generally guide me one way or another. As a couple of concrete examples:
- For a recent DIY UPS build, I used SLA batteries. They're sitting on a metal rack and will be charged full for nearly they're entire lifetime. For this project, I had an inverter that was preset for SLA high and low voltage limits. I just needed to buy an SLA charger. Because I will cycle them rarely, I did capacity planning at about 30% depth of discharge. Yes, that's more than the 50% rule. It will harm the batteries, but the situations I'll need that much discharge are rare and may never happen before I replace the batteries several years from now. Price mattered to me, it was a personal project and I wouldn't have done it if I had to spend much more.

- For a recent mobile battery box, LFE was the better choice for me. For the capacity I wanted, LFE was the only reasonable choice. It's still almost too heavy. I don't cycle these like a typical home solar install, but I do use it more in various situations than the UPS above. I would prefer to keep this 100% charged all the time, but I keep it about about 80-85% to get me a little more lifetime on the batteries.