Choosing a small deep-cycle battery for portable power backup

[LMF5000]

I'm building a makeshift portable "generator" for powering essential appliances during power cuts (basically a modem(5W), desktop PC (200W), fan (40W) and possibly fridge (160W) on occasion). It will essentially consist of a battery and a 500W (continuous) pure sine wave inverter that I carry around the house as needed.

Based on being able to run the fan (most essential load) for a 4-hour powercut, energy needs are 40W*4h = 160Wh. Factor in 90% inverter efficiency and 80% depth of discharge and you get battery capacity of 160 / 0.9 / 0.8 = 222Wh. Divide by 12V and you get 18Ah. Add a bit more for safety and it means 25-40Ah of battery should do it. I'm targeting battery weight of under 10kg to make it easy to haul around. The battery will be stored disconnected and charged infrequently. It will be used every few months when there's a power cut, for however long it takes for electricity to come back (2 hours on average).

I tried looking into LiFePO4 batteries, but most good-value online companies don't ship to Malta (Europe), and all the logistics companies wouldn't touch lithium batteries with a 10-foot pole (dangerous goods, ADR declarations etc.). A lot of them claim they're working on giving me a shipping quote and then... silence.

So I'm basically stuck looking at locally-available small/medium-size deep-cycle lead-acid batteries. The thing is, there are a lot of choices with similar prices and specs. Especially within the same company's own lineup, they offer multiple series with cryptic names (such as "CP series" or "CPL series") but the product description is more marketing lingo than a technical description of, say, what makes the "CPC" series good for cycling applications while the "CPL" series has a longer life and "CPR" have high vibration resistance for railways...

I was wondering if you battery experts could help me identify the best battery for my application, and maybe explain a bit how they design a battery to have long life, or high cycles, or high discharge, and what makes a "traction" battery.

Here's my shortlist of the four locally-available batteries under consideration (prices are local prices, links are to manufacturer page for that model):
1. Victron SuperCycle AGM 24Ah. €100. Battery description here, datasheet here.

2. Cellpower CPL 28-12IA long-life AGM 28Ah. €130. Link

3. Cellpower CPC 33-12 cyclic AGM 33Ah. €133. Link

4. pbq HR22-12 high-rate AGM 22Ah. €94. Link

I'm most impressed by the Victron in terms of company literature (#1) but the Cellpower long-life (#2) seems to be the best fit for my application. Thoughts?

 

[itsmoked]

Man.. Where to start. For starters if you're going to invest the time provide at least enough storage to make it worth your while.

Have you looked around a little? For starters, if you are carrying this around skip lead acid entirely! You can't get enough out of a human carryable lead acid battery to do anything. Keep in mind you can't draw a lead acid battery down below 50% SOC (State Of Charge) without damaging it. This means right off the bat your storage will weigh about 4 times the same available energy in LiFePO4 technology.

I suggest you consider something like this. Perhaps smaller (or maybe not) but the same setup. It's the biggest bang for the buck you'll ever see.
Hand Truck Solar System

You can't go wrong taking in a bunch of Wil's videos. His book is good too. He comes from years living in an RV and running it all solar.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LMF5000]

Hi Keith,

I've been looking around for around for at least the last 4 weeks. I must have contacted 20 different battery and logistics companies. LiFePO4 batteries are as rare as hen's teeth here. My only choice would be Victron models: a 20Ah 12.8V SuperPack is €300, next step up is 60Ah for €600. That's 6x the cost per amp-hour of lead-acid.

The budget for this is €150, and it will get very occasional use (basically a few times per year) - so since many lead-acid models claim 300 cycles even at 100% depth of discharge, that still represents a good 10 years of use in this application. Will's setup is very nice, but he's in America where LFP batteries are easily obtained and reasonably priced. In Malta I'd have to have them specially shipped to me at great expense.

I know what you're saying about performance, I do operate lithium polymer batteries in my R/C projects and 18650s in everything else (I even built a 3s7p custom pack from recovered laptop 18650s by carefully matching real capacity and self discharge), but in this application it seems putting down the extra money for LiFePO4 would just get me capability that I won't be using.

 

[itsmoked]

Hmmmm Malta. I see...

Ho Kay. I live in the land or PG&E, the company that has a mass murder rap of 84 people. So my power is often questionable. Lately it's down right weird with outages that are longer than ever expected and in the increased global temps more frequent. I suggest you double your estimates for the future. I consistently find I under estimate the required storage even after careful auditing.

I think you should consider a quality automotive flooded battery. Due to their production volume, ubiquity, and competition, you will get probably 3 times the capacity per dollar than you're seeing with your little herd of choices. Can you leave it somewhere protected like in a closet, garage, or outside? You then place the inverter and charger with it and only carry around an extension cord to where you want the power.

Consider something like what Wil showed in that video only using your LA battery. Then you can cleanly mount it all on a little hand truck or small dolly. You could use a milk crate design bolted to a dolly. Etc.

A typical small car battery (Group 24) is about 80AH.
If you insist on carting this stuff around the house get one with a carrying handle. Many have them built in.

I would put it on something like the aforementioned dolly in a "Battery Box" like the ones used on boats to keep the fizzies at bay and preventing anything from accidentally coming in contact with the terminals.

Use a smart charger that cuts off or goes to float. It could be as small as a 1A charger. Always recharge the used battery the very moment you can after usage. This will maximize the life. Then make sure you toss the charger on at least every month as LA discharge about 1%/day in 30C weather. If you do 90% discharges don't expect more than about 60 cycles on modern batteries. If you leave a battery that discharged 24 hours before recharging expect something more like 20 to 30 cycles.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[LMF5000]

Actually I'm currently using flooded automotive batteries for the task (basically when my car battery will no longer start the car, I buy a new one and use what's left of the life of the old car battery). The problem with that is that they're bulky and heavy and can only be discharged 20% before starting to do permanent damage to them. I wanted something lighter and smaller and with more usable capacity. The battery will be stored indoors. Problem is it's a warm country and room temperature is a steady 25-30°C for 6 months of the year so battery life shortens somewhat.

Will's idea is great, but I already have the equipment to mimic it at a much smaller and simpler scale. No BMS needed for LA, and the inverter I chose has voltage and load readouts (though I also have a DC power meter with coulomb counting which I can put inline to keep track of battery capacity). For charging I have a hobby charger that can handle most chemistries (Ni-Cd, Ni-MH, Li-Po, Li-Ion, LiFEPo4, LA). It's a Charsoon Antimatter 20A/300W that's derived from the iCharger 206B. Can read internal resistances, charge, discharge, storage-charge (3.6V for Li-ion) and balance charge (up to 6 channels). The lead-acid cycle is CC-CV with settable current and voltage limits. Termination is when current in CV decays to 10% of the current limit set at start (i.e. if I set 15A 6s @ 2.4V/cell it will do a CC stage at 15A until 14.4V then hold at 14.4V until current falls to 1.5A). There's a trickle charge mode, but it's not CV like a true float charge, it's CC like you'd use on a nickel-based chemistry (fixed at 60-200mA). So I don't actually float/trickle for LA, I just do cycling.

 

[LMF5000]

Since you're a battery engineer I wonder if you could advise the proper voltage for charging Ca/Ca maintenance free car batteries? I haven't been using the car due to lockdown and the battery fell flat about 4 times in as many months. I charged it up to 14.4V a few times and kept it operational, but the last time I tried charging it up to 14.8V as I read on some sites that that was the proper voltage for calcium grids. It developed a shorted cell shortly thereafter and I couldn't revive that cell (SG was practically 1.0 regardless of how long I kept forcing current into it). I recycled the battery now, but is there anything I could have done to revive it?

I've made sure not to repeat the mistake because the new replacement car battery is an AGM stop/start battery, and I have a small 5W solar panel hooked up to the battery via a DC-DC boost converter set to 13.2V (open circuit measurement). In practice it keeps terminal voltage at 12.8-12.9V when the sun is shining. Should help deal with the parasitic loads of the car (which I measured to be 0.04A).

 

[itsmoked]

Nothing can be done to reliably rejuvenate car batteries. You simply cannot let them go flat as it destroys them. Letting then become fully discharged ~12V and not IMMEDIATELY recharging them is immediately damaging to them to some extent so letting them go flat (0V) destroys them. You need to run a modern parasitic car weekly to keep the battery healthy. OR put a solar charger on that keeps the battery floated at its float voltage and should keep the battery at it's float voltage not 12.9V.

Keith Cress
kcress -

http://www.flaminsystems.com