How to determine battery capacity?

Topic: How to determine battery capacity? (Read 871 times) previous topic - next topic

0 Members and 1 Guest are viewing this topic.

Re: How to determine battery capacity?

Reply #25 – February 17, 2018, 03:48:48 pm

That makes sense Andy.
Thanks.

Re: How to determine battery capacity?

Reply #26 – February 17, 2018, 05:40:14 pm

"You probably know that a big [gotcha] for lithium is don't let them get hot”

Yes, I’ve read that. The datasheet for my Victron Smart Lithium batteries says the operating temperature range is -20° C (-4° F) to +50° C (122° F), but I don’t intend to let them ever get that hot. For one thing, they’re located in a ventilated space under my bed, and I’d never let my bedroom get that hot! For another, they have enough thermal mass that their temperature stays fairly stable, hovering around 60° F. in the month or so I’ve been using them, even as inside temperatures ranged from the upper forties to low eighties. In any case, my charging system will automatically cut off all loads if the temperature gets too high.

Low temperatures are another gotcha, but it’s a special case: you can draw power from them down to -4° F, but you basically must not charge them at temperatures below freezing. (Victron says, “maximum charge current when battery temperature < 0° C: 0.05C, i.e. 10 A in case of a 200 Ah battery.”) I don’t have automatic protection against this scenario, but again, my bedroom never gets that cold.

(Bear in mind, though, that I live here. If I were storing the rig for part of the year, I’d probably disconnect the batteries to protect them from temperature extremes.)

The third gotcha is the need to keep the cells balanced with each other. In my case, Victron has built balancing circuitry into each battery, so it’s taken care of automatically. My batteries’ cells stay with ten millivolts of each other.

“It was a screaming 204 watts - four 51-watt panels”

Thanks for the correction, Judie. Yes, the odd number had slipped my mind. 200+ watts was almost unheard of in a small class C back them, and I was very lucky to get such a technology-loaded rig for my very first RV.

“Just put my 2cents in the LDO Amazon kitty by ordering ‘Mobile Solar Power Made Easy’ ”

Let us know how you like it, Kent!

“the Victron lithium battery system and its matching accessories [are] marine grade equipment and should hold up to the RV environment.”

Yes, I bought Victron’s lithium batteries, battery management system, load and charging cutoff relays, solar controller, and of course battery monitor. I chose not to get their charger or inverter because everything they made was too tall to fit under my bed. Instead, I installed a ProNautic marine charger (top of the line equipment) and a 1,500 W Xantrex inverter (nothing fancy, not even pure sinewave).

Victron’s equipment is very well made and their tech support is quite responsive. I was a bit disappointed in their documentation—while slick-looking, it doesn’t have the level of technical detail that can be found in the Lifeline Technical Manual.

What I really love about the Victron equipment, as I’ve probably said elsewhere, is that everything is Bluetooth-enabled and can be monitored and controlled from their well-designed iPhone/iPad app. That meant I didn’t have to find space for a collection of wall-mounted monitor panels, nor run wires to them. Everything I need to know is in one place on my phone, with more useful detail and historical data than any wall panel can show at once... all available at the swipe of a finger. Even the batteries have built-in Bluetooth—I was able to check their condition while they were still in their unopened crates on the loading dock!

All this convenience came at a cost. I have friends who put together lithium battery systems for less than half what I spent. They bought cells from China, assembled them into battery banks, added support systems, and so on. They have display panels that let them monitor what’s going on. When cells need “rebalancing,” they do it manually. With Victron components, I took the easy, but expensive way out. It was the right way for me, but not for everybody.

Note to those with sharp eyes: the attached screenshots of the VictronConnect app were taken at varying dates and times under varying conditions, so don't expect them to be consistent with each other.)

Re: How to determine battery capacity?

Reply #27 – February 18, 2018, 02:41:19 am

Quote from: Andy Baird - February 17, 2018, 05:40:14 pm

All this convenience came at a cost. I have friends who put together lithium battery systems for less than half what I spent. They bought cells from China, assembled them into battery banks, added support systems, and so on. They have display panels that let them monitor what’s going on. When cells need “rebalancing,” they do it manually. With Victron components, I took the easy, but expensive way out. It was the right way for me, but not for everybody.

Considering how easily it is to kill a set batteries with improper charging, no matter what type they are, it's a smart move to buy a well engineered and proven system.
You are in good company, AM Solar is now installing Victron lithium kits, instead of their building their own.

It might be interesting to check back with your friends, in a couple years, and see how their home-brew systems are holding up.
On the other hand, ten years ago you might have been willing to build your own kit too, if the parts had been available.
I guessing that after 15 years of messing with batteries and chargers, you are ready for a power system with less drama.

Larry

Re: How to determine battery capacity?

Reply #28 – February 18, 2018, 05:21:13 am

"guessing that after 15 years of messing with batteries and chargers, you are ready for a power system with less drama."

Yup! True, most of the "drama" back then was due to my ignorance. But when I made the jump to lithium*, which is still a relatively new technology in the RVing world, I wanted a system that "just worked"--not one that I'd have to constantly monitor and tweak.

* technically, lithium iron phosphate (LiFePO4)

Re: How to determine battery capacity?

Reply #29 – February 18, 2018, 07:53:20 am

Back in 2013 I installed a 1000 AH LiFePO4 system in my Newell. I bought 4 large cells from Balqon (who unfortunately is now defunct) along with their BMS system. I used a 12 volt spotlight to bring each cell down to equal each other. They ranged from 3.446 to 3.481 when I received them, which is not bad but 3 years ago I sold this system to an engineer buddy who is still using them in his coach. He recently told me they are still within 0.002 volts. So the BMS is doing a great job of internally balancing the cells. And after 5 years they are still holding their original capacity.
I would expect your Victron cells to last even longer since Victron has a greater R&D and quality control.
It was amazing to watch the system voltage when running the large microwave. It would only drop 0.2-0.4 volts! compared to 1 volt or more with the 6 8D wet batteries.
LiFePO4 is an amazing battery technology.
Here is a link to my install: Lithium battery install

Re: How to determine battery capacity?

Reply #30 – February 18, 2018, 01:09:34 pm

"Here is a link to my install"

My hat is off to you, sir. That was a massive (by LD standards, anyway) 1,000 Ah installation, and your detailed descriptions were very enlightening. Tackling that project back in 2013 on a DIY basis, using parts from a company with little to no customer support, took courage. (My friends also used Balqon cells and had the same complaints.) Well done!

Log in

Lazy Daze Owners' Group

Reply #25 – February 17, 2018, 03:48:48 pm

Reply #26 – February 17, 2018, 05:40:14 pm

Reply #27 – February 18, 2018, 02:41:19 am

Reply #28 – February 18, 2018, 05:21:13 am

Reply #29 – February 18, 2018, 07:53:20 am

Reply #30 – February 18, 2018, 01:09:34 pm

Affiliate Notice