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Old 03-12-2015, 05:31 AM   #1 (permalink)
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Need a new LiFePO4 Battery

I'm an idiot.

I swapped out the OEM 45Ah 12v battery in the Prius with a 4.2Ah LiFePO4 battery to test the charge/discharge efficiency vs the stock battery and see if it's a suitable replacement. Then I decided to explore Techstream (Toyota diagnostic and service tool) with the car on acc mode (accessories on, but power off). I spent half an hour with the door open and the radio on while looking at the various settings I could change. I turned the annoying seat-belt warning for the passenger seat off.

Then suddenly the radio cut out. I was confused for a moment, and then realized I had probably drained the small battery too much. I hit the power button to turn the car on and begin charging the 12v battery, but it was too late. There wasn't enough juice to turn the car on.

By the time I got to the battery and disconnected it, it was very warm and puffy. The meters I had connected to it showed 6 volts; not good!

After attempting to balance charge, one of the cells won't come up to proper voltage. The other 3 cells appear to be fine, but cell 1 appears to be fatally wounded.

Now I'm looking at replacement options, and I'm pretty sure I want to go with a modular design, such as prismatic cells, so that I can replace weak cells. I'd be interested in anything from 4Ah to about 20Ah.

Any recommendations on where to get the best price per Ah? I realize I'm not going to get any discounts by purchasing only 4 cells, but I figure someone on here would know of a good place to buy.

So far I'm leaning towards 20Ah prismatic cells from batteryspace.

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Old 03-12-2015, 08:00 AM   #2 (permalink)
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Maybe use one with a PCM?
It can't run a starter motor but should be fine in a hybrid if that starts its engine on the hybrid battery.
EV-Power | LiFePO4 Battery Pack (12V/12Ah PCM)

Or maybe a slightly bigger one without PCM:
EV-Power | Lithium Battery LiFePO4 (12V/20Ah)

You can find scores of cheap batteries on Ebay and Alibaba etc, but frankly I'm scared to try those myself. In some cases the specs can't be right, like when the weight of the battery is less than it should be for the indicated Ah, or "LiFePO4" with 3.7 volts per cell...
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Old 03-12-2015, 09:25 AM   #3 (permalink)
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If you have a smart key, you want some extra capacity as that will slowly drain your battery. I think the stock LA is around 40 Ah IIRC with the smart key, its only around 30Ah without.

Other than that, its not a real demanding application at all. The 12V boots the system and thats about it. The DC-DC converter takes over from there and handles all the 12V stuff. The 12V is just a buffer at that point.
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Old 03-12-2015, 02:49 PM   #4 (permalink)
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I measured the Prius with smart key parasitic draw at 13mA. Still can't believe I killed the battery. By my calculation, I was drawing just 5.6 amps for those 30min. Should have been no problem for my 4.2 Ah battery, but then again it may not have been fully charged to begin with.

I'll probably go with the 20Ah prismatic cells for $35 each. Keeping the system modular is important to me so that a single failed cell doesn't brick the whole battery. The larger capacity should be a good buffer and handle abuse better.
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Old 03-12-2015, 03:19 PM   #5 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
...Now I'm looking at replacement options, and I'm pretty sure I want to go with a modular design, such as prismatic cells, so that I can replace weak cells. I'd be interested in anything from 4Ah to about 20Ah....[/URL].
Between 4 and 20Ah ?. May I suggest 19.6 Ah

I use 4xA123 19.6Ah in my Prius 2 since one year with a capacitive bms.
Still ok. But I avoid using this 13.2 volts without the prius dc-dc converter on.
This battery is 8kg "ligther" and can be charged by the prius dc-dc-converter down to -10C -15C and up to 40-50C. Just after power on you can have 30A falling to 20A quickly.


On the prius gen 2 the max voltage is 14.1 volts. So 3,52v for each cell when well balanced.

In the prius there is no 12v battery low voltage cut off.
All what is connected via a relay will have a 9-10v protection.
I replaced all the w5w bulbs with led, say 4v protection.



Are you sure your 4 cells had the same capacity ?
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Old 03-13-2015, 03:13 AM   #6 (permalink)
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Quote:
Originally Posted by planetaire View Post
...with a capacitive bms.
Still ok. But I avoid using this 13.2 volts without the prius dc-dc converter on.

Are you sure your 4 cells had the same capacity ?
What is a capacitive BMS? Is it more efficient than other schemes? I'm also unclear about what you mean by avoiding the 13.2 volts without the Prius DC converter on.

I'm not sure the 4 cells had the same capacity because I might have damaged the battery once before while trying to measure electrical loads on the Acura. Testing took quite a long time, and I was going through each accessory load and ended up running the voltage too low, to maybe 8 volts. It got very warm, but I put it on the balance charger and figured it was ok.

This time the voltage dropped to about 6 volts, got very warm, and the battery is puffy. Cell 1 refuses to go above about 2 volts on the balance charger, and rapidly drops below 1 volt when left alone.

I'd be interested in a low voltage cutoff, but those probably put a large electrical load on the circuit themselves.
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Old 03-13-2015, 09:38 AM   #7 (permalink)
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Active bms. 13.2volts

There are two main bms familly:
-passive : bleeding
-active : capacitive, inductive, dc-dc transfer ...

I don't use bleeding because when cells goes up to 3.65V it is "too late".
Amperage during charging can really goes up to 100A and cannot be cut. With lifepo4 near 3.65v voltage goes up very quickly. May be a good diyer could fool the hsd and stop charging. On the 12v plus pole there is a sensing wire (a small one), just to read the battery voltage. May be if a device increase this voltage the hsd will stop charging the 12v battery ?

So I use an active bms, a capacitive one, that transfert energy from the higher cell to the lower. It does this permanently, 24h/24h 365 days/year.
So when I drive my prius, at the begining of each trip the 4 cells are always balanced, often within only 0.01 volts difference between the 4 cells.
There is a small voltage difference at the begining, just during the initial charging and after, during the trip, the bms rebalance them.

In winter you have to be careful. Some lifepo4 may be charged only over 0C, others under.
And charging amperage can goes up to 100A (I have seen such an amperage with a lead acid battery in a prius), so in winter avoid discharging your cells during power_off or accessories positions.
Also check that you cells accept such an amperage. With 20Ah cells that is 5C. Usually it is 1C and more during few seconds, but you have to buy cells with 5C charging, it could happen.

13.2 volt is the rest voltage, in power_off mode. When well charged (and they are, 3.52v during power_on is near 100% charge), after waiting several minutes, the A123 have 3.3v. Others lifepo4 have 3.2v.

Low voltage cutoff may be worse then not having one. It must be 100% safe and as you say it can use some energy and drain the cells.

I was only talking about the gen 2 Prius. The gen 3 can have a highter voltage during charging (something like 14.7 volts ?)


May be, if one of your cells don't have the same capacity or was not charged as the 3 others, may be its voltage goes to 0 volts and not the 3 others cells.
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Old 03-13-2015, 08:10 PM   #8 (permalink)
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Have you seen this review at endless-sphere?
Endless-sphere.com • View topic - Battery Space 20AH Prismatic LiFePO4 tests
In that thread I noticed this link for a 4S 20AH pack from elite power solutions, Elite Power Solutions
The GBS cells look good, if my 12V Headway aux pack in my Geo Metro EV dies, I might go that route.
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Old 03-14-2015, 11:51 PM   #9 (permalink)
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You may want to try contacting this guy about some cells. Don Blazer at theoldcars@aol.com. I haven't bought any but I know a guy who has and was happy. I did get a price quote once and it was reasonable, plus he is in your State. Advocate Pricing On CALB New CA Cells All Sizes - DIY Electric Car Forums
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Old 06-21-2018, 08:40 AM   #10 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
I might have damaged the battery once before while trying to measure electrical loads on the Acura. Testing took quite a long time, and I was going through each accessory load and ended up running the voltage too low, to maybe 8 volts. It got very warm, but I put it on the balance charger and figured it was ok.
Getting warm indicates dendrite growth (internal short). It happens when you recharge too fast at low voltage. No damage is done during discharge (unless you leave it there for weeks)

If you overdischarge, recharge very slowly and you will have minimal damage

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