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@Dominic

I've finially received my new batteries. Thanks to @neftaly who has a family hookup to get things imported very cheaply from china, I got 200 AH LiFePo4 for $340 NZD. That is about half the price a lead acid, plus you can discharge these batteries deeper than deep cycle, so in practical terms (not marketing) they actually hold more usable energy.

@neftaly gave me some advice for managing the batteries charge manually, reposted here with his permission:

I took my BMS out entirely for now, going to just manually maintain them until I get my new one. As long as you never let any individual cell go over 3.65v (and avoid discharging to under 2.8v, which isn't as bad as overcharging but still bad), you should be golden. Run a multimeter over each cell and try and keep the voltages the same (you can do this while they're still connected together in series). Do you have the ability to limit your charger to 14v? Also a cut-off switch for your alternator would be helpful.

They will mostly self balance as you take power from all of them when in series (higher voltage cells will have more power taken from them, so will drain faster, and return to equilibrium). But if one gets out of whack, you can just put a load on it to drain power, such as a 12v incandescent bulb across the terminals, until the voltage drops enough.

If one gets super discharged, you can try disconnecting everything, then reconnecting everything together in parallel (all positives connected together, all negatives connected together, nothing else connected) and leaving it for a while. Power levels will gradually equalize, though it gets exponentially slower as the voltages get closer together.

Be very careful when you're installing the battery bus bars and stuff - it's sooo easy to accidentally short circuit it when you're using a spanner. I wrapped mine in electrical tape to help.

The first time you connect solar etc, keep an eye on it as voltage hits the limit (use a multimeter, don't just trust the votlage readout). You wanna make sure your solar charger cutoff actually works as expected.

@nichoth
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@dangerousbeans
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@dangerousbeans

Cool, how heavy are they? Do they look like spaceship engines?

@Dominic

They are not that heavy they are 4*3.2v cells, I was able to kayak across the harbour with them in their box between my legs. Okay it was awkward, but I made it okay. You are not gonna hurt your back lifting them, especially since they are multiple cells!

Oh, they look exactly like these ones: https://www.ev-power.eu/Sinopoly-40Ah-300Ah/SP-LFP200AHA-Lithium-Cell-LiFePO4-3-2V-200Ah.html

Says 5.6 kg per cell which sounds right, so 24kg of battery in all.

Next I need to order something like this: https://www.ev-power.eu/SBM-CBM-1-1/Simple-Battery-Management-Board-4-cells-12V-16A.html which prevents overcharge. Unlike a lead acid battery you need a device like this to make sure one battery does not take all the power.

@piet

Not running a charge controller sounds pretty scary to me.

Apparently LiFePO4 are a bit more stable than LiPO but still.

I had a RC chopper and I remember reading about the dangers of those batteries. The phrase "same energy density as a hand grenade" has stuck with me. Not sure how true it is but it made me a lot more careful.

If one gets super discharged, you can try disconnecting everything, then reconnecting everything together in parallel (all positives connected together, all negatives connected together, nothing else connected) and leaving it for a while. Power levels will gradually equalize, though it gets exponentially slower as the voltages get closer together.

That makes me bloody nervous. The internal impedance of these types of batteries is incredibly low. Even if you have very little voltage difference, very large currents can flow and violate the absolute maximum charge / discharge rates of the battery.

I think balancing with a load is way safer.

@Dominic

@piet clarification: I still have the MPPT charge controller, and for lifepo4's in series happens to have the same voltage as a lead acid, so that works.

What is missing is just the balancer.

@juul
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@piet
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@neftaly

@Piet Unfortunately, there's a lot of FUD regarding LiFePO4, understandably brought about by the reputation of LiPO. In actuality, they're even safer than lead-acid. Worst-case failure conditions LiFePO4 mean venting toxic smoke (they don't heat up enough to catch fire). Worst-case for lead acid is exploding (H2 + O) and spraying H2SO4 around.

https://www.youtube.com/watch?v=aQs7L5LmEss

Regarding balancing in parallel, the voltage differential is low by their nature, and their charging limit is 3C-5C (600a-1000a). You'd need some impractically thick wires (and a battery so flat as to be already ruined) to get anywhere near that! It's too time-consuming to be practical for regular balancing, and more intended as a way to recover from fuck-ups.

I prefer a BMS because I'm lazy, but they're absolutely not a necessity, and there are plenty of people running fine without them :)

@neftaly
  • and to make it clear, putting a <2.8v LiFePO4 battery in parallel with a tonne of others and no current limit is only a good idea if the alternative is holing on rocks because you can't get your engine started
@Dominic

I have the power that comes to my laptop on the MPPT's load terminal. I've set this to depower when the battery gets to 12v, that should keep me well above the 2.8v no go zone.

I've also noticed that the voltage reading at the battery terminals is higher than at the charger's battery terminals... Guessing I need much thicker wires here?

@Dominic

My MPPT charge controller was showing low voltage and shutting down the load (my laptop) but at the battery terminals I still saw 13.18 volts (lifepo4 decrease in voltage only very sligtly as they discharge). I was loosing 2 volts somewhere! So, I rewired from the controller to the battery directly, and now it's happy. I wonder if this means my old batteries where not as bad as I thought?

@piet

@neftaly
Cool! Sounds like LiFePO4 are way better than LiPO.

I had a different intuition about what it might take to break those batteries. So I did some calcs for fun (this stuff used to be my job so it was kinda for my fun, not trying to win an argument)

My question was: could you violate absolute maximum ratings of these batteries that @dominic linked to.

TL;DR: Yes, with two 7cm long (distance between terminals of adjacent batteries), 2mm diameter wires and a difference in battery voltage of larger than 0.3V.

Max discharge is 600A, but max charge is 400A.
From this wire resistance calculator, 2 * 7cm wires of 2mm diameter wire is 0.000747 Ohms, or 747uOhms.

What potential difference do we need for 400A over 747uOhm?
V = 400 * 0.000747
=> 0.298V

Surprisingly low!

Also for fun, those wires would be HOT
P = I^2 * R
= 120W!

Not trying to start a "I'm right you're wrong" war. This is in the spirit of mad scientist, back of envelope, where's my damn calculator fun.

@Dominic

thanks @piet so are you saying that if I need to do the parallel voltage rebalancing thing, use very short fat wires?

Also, now that it's wired directly to the charge controller, I've done about 6 hours of computing since the sunset and level hasn't budged! A good sign but the real test will be being anchored waiting out some bad weather with several days overcast!

@xj9
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@ezdiy
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@Dominic

~ $240 for 200AH is a very nice deal for lifepo, these things are practically immortal if you 30-40% DOD.

The only thing I'd be worried is well ... china (aka reputation of the fab it came from)

@cryptix
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@piet

@dominic No the opposite.

That calculation showed it might be possible to have damaging large currents flow if:

  • There's a difference in battery voltage larger than 0.3V.
  • You used short wire larger than 2mm diameter.

I guess I'm saying: The parallel voltage re-balancing thing could damage an otherwise fine battery so don't do it. Get the right tool for the job (a load balancing charger).

@Dominic

@piet awesome, thanks!

@ezdiy I'm bullish on china. Remember when people thought everything japan made was crap? (I'm not actually this old, but...)

@nanomonkey

@Piet I believe you are disregarding the internal resistance of the batteries themselves. A LiFePo battery tends to have a internal resistance around 0.1 Ohm, which means that the current drops to around 2.97 Amps for a 0.3 V difference.

@piet

Hey,
Nah, I thought about the internal resistance. It has to be WAY lower that 0.1Ohm. Like ~250 times lower.

The batteries dominic linked to are rated for 3C continuous discharge. That's 600A.
If those batteries had 0.1Ohm resistance, the voltage drop across the internal resistance would be 600 * 0.1 => 60V. Which isn't possible for a battery with 3.2V nominal voltage.

Put another way, the most they could give short circuit when fully charged would be 3.7V / 0.1R => 37A.

Where did you get that 0.1Ohm figure from?

I looked at the datasheet and it says:

  • The internal impedance @1khz is <= 0.4mOhms
  • If I calculate it from the graph I get 0.6mOhms but it's hard to read exactly.

Either way those batteries can deliver a hell of a lot of current! :)

@Clayton Koenig

The only thing I'd be worried is well ... china (aka reputation of the fab it came from)

I would be inclined to second that. I don't know anything much about batteries, but living and shopping in China (and buying a lot of stuff on taobao.com) I would say that buying anything at the bottom of the price range is asking for very poor quality kinda trouble. Think the pair of sports pants I bought years ago, and tore the crotch out of from waist band to waist band the first time I tried to kick in them. One wear. On the other hand, you don't have to spend a lot of money to get quite good quality. Just stay away from the very bottom of the price scale, where all the crooks hang out.

The funny thing about China is that even though at high levels of visibility laws and regulation can be very repressive, at low levels it is the wild west and there are basically no laws and regulations. And the transition from low to high is precisely where a lot of people here find themselves thrown into jail, or worse.

@nanomonkey

@Piet The 0.1 Ohm was from a quick internet search. If they are 0.4 mOhm that would definitely change things. At that internal resistance, the current would be around 401 amps which is just above the maximum charge rate. I'd assume, like most batteries, that the internal resistance goes up a fair bit when the battery is at the end of it's discharge cycle, so it's possible that the amperage would be below this.

Fun.

@keks

Doesn't rewiring for balancing suck anyway? I would assume they get unbalanced within one charge, and rewiring on every charge is just not workable - at least I think so. But then, it seems a lot of people do it that way...

Here is a paper giving a nice overview of several techniques for balancing: A Review of Passive and Active Battery Balancing_ based on MATLAB-Simulink.pdf

The simplest design is to connect a large resistor in parallell to each cell. I guess it looks like it's not very efficient though. I tried calculating the losses but the last time I did EE is several years back and I'm not 100% sure I can treat a battery as a simple voltage source.

Any electrowizards around?

@Dominic

@keks it's just a temporary measure, if necessary.

@neftaly

@Dominic Last night I started my engine, attached a multimeter to the alternator, turned on some lights (to give it a load), then disconnected my battery. Adjusted the trim pot on the alternator voltage regulator till I got 14 volts. Seems to have worked! It's nice that they use nothing more than a rectifier and a cheap regulator for charging, instead of a more complex "smart" multi-stage charger.

@Sybil They're Sinopoly, which seems to have a good rep (watch the youtube video of them I posted earlier, it's my favorite!).

@interfect
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