Charge Amps for LiFePO4?

[HRTKD]

In Will's charge profile post (https://diysolarforum.com/threads/r...le-for-diy-lifepo4-batteries-sticky-post.5101) he covers volts. But what about amps?

The converter in my RV is specific to lead acid batteries and isn't programmable, but it can charge at 45 amps. Consequently, I'm looking for a new converter. I'm not sure what amp rating to go with.

The Victron Skylla-IP44 appears to be capable of up to 60 amps and has a LiFePO4 charge profile. However, it's very expensive.
The Victron Blue Smart IP67 has some nice features but as near as I can tell, it goes up to only 25 amps. Much less expensive.
Progressive Dynamics has multiple chargers that support a LiFePO4 charge profile, ranging from 30 to 80 amps. More expensive than the one above.

The 5500w generator on my RV can handle any of those. When I'm using the generator to charge the batteries it's because there isn't enough solar and charging quickly is ideal. But is 80 amps acceptable?

I will have the EVE 3.2v 280Ah cells, either in a 2p4s (single BMS) or a 4s2p (one BMS for each battery).

 

[Forbisher]

You can charge your LFP battery at 0.5C so 140A into a 280Ah battery.

 

[Bob B]

Charging a 280 AH battery at 80 amps would be less than .3C and if you are parallel at 560 it would be less than .15C ..... so, there should be no problem charging at 80 amps as long as your wires, fuses, and connections are up to it.

 

[HRTKD]

It sounds like I need the higher amp converter then. I'm disappointed that Victron doesn't have a product in the 80 amp range.

 

[Steve_S]

Not pointed out.

The BMS you have on your packs can also be a limiter as to the amperage ou can push to the battery pack. If for example, you have a 100A Fet Based BMS, the charge capacity is likely throttled to 50A max through the BMS. If you are using a BMS with Relays/Contactors, then your limit is set by the maximum Amps the relays can pass through. Choosing the right BMS for the Charging & Discharging environment and the cells capacity is a pretty important step. PS, always better to over-estimate capacity by a minimum of 25% to leave elbow room. ** ALSO you need to take into account the potential Surge Amps that can be demanded by the Inverter. An A-Typical Pure Sine Low Frequency can pull 3X the standard rating for surge, so a 4000W Inverter can pull up to 12,000W for surge handling. That is 1,000A @ 12V, 500A @ 24V, and 250A @ 48V.

With LFP figuring out how much you can push is actually pretty simple. Common LFP will allow for 1C. Meaning a 200AH cell can accept 200A for One Hour, it can also usually discharge at 1C or 200A. If LFP is pushed to Max Charge/Discharge Capacity, the cells will get warm, quite warm to hot actually. It is also hard on the cells and therefore not recommended. There are Hi Output LFP cells, capable of 3C or higher and some with Ytrium for Cold Temp functionality but now your deep into the wallet.

I use a Midnite Classic 200 SCC which for 24V can output 79A from my array. Retail cost just for the controller is around $900 USD These can be stacked to increase that. Many solar controllers can do so (not the cheapo value ones though).

My Inverter is a Samlex EVO which can take Grid & Genset AC to charge while allowing for passthrough to house. Unfortunately, even with that, it is limited to a max of 50A output to the battery bank. The "average" for most Inverter/Chargers is around 50A maximum, some go higher but the price becomes exponential. Some Inverters are also "stackable" as such, so that is another avenue for large systems.

I cannot recommend paralleling large LFP cells as they diverge & fluctuate a fair bit during charge/discharge. Dealing with hundreds of AmpHours per cell. I suggest building 2 Two 12V/280 straight 4S packs with a BMS each and parallel them together. That will allow you to manage each cell within a pack and will make each pack an independent entity capable of carrying on if the other stops for some reason.

BTW: Samlex Inverter/Chargers were designed for RV/Marine & Mobile use and are often used as replacements for RV "Converters" and so many of their features & functions are there to accommodate such like built-in Auto-Transfer Switch for AC inbound, they are very programmable and configurable and even have a Solar Port built into them to accept up to 50A Solar Input from an SCC directly to the Inverter/Charger.
Check out the links in my signature for more info.
Or check at Samlex: https://samlexamerica.com/products/ProductDetail.aspx?pid=575

 

[RCinFLA]

BMS cannot 'throttle back' charge current as an implied variable adjustment connotation. Just so no one gets the wrong impression, it can only connect or disconnect the battery from load or charger.

If your BMS MOSFET switches are not low enough equivalent series resistance (high enough BMS current rating) they will get hot with high current. Assuming BMS has a temp sensor to prevent it from destroying itself the temp sensor will cause the BMS to open the MOSFET switches to save them from destruction. When it disconnects, the MOSFET's will cool down and reconnect again after a few minutes. Most of the time this happens because of too high discharge current due to high inverter load.

So the process of connecting and disconnecting duty cycle can reduce the average current. This is not a desired way to operate however as there can be a large surge current associated with reapplying power to an inverter. An inverter often remains in standby after applying power until user reactivates it so this on-off 'duty cycle' modulation will not occur on its own.

There is also an immediate high current sense shutdown but that is triggered by much higher current and is more like a circuit breaker for shorts or inverter failure.

I also prefer not to directly parallel cells together, instead have multiple parallel strings with their own BMS. Paralleled cells need to be closely matched for equivalent internal series resistance and capacity. Manufactures like BattleBorn or Tesla do this by ensuring batteries are taken from same manufacturing lot supplimented by some additional testing that is not likely available to a DIY individual with limited supply of batteries to chose from.

 

[HRTKD]

The last two posts may have put the nail in the coffin for my 2p4s configuration. If I have separate 4s batteries, each with a 120 amp rated BMS, an 80 amp converter shouldn't be a problem, right? The BMS from OverKillSolar says it can do 120 amps charge/discharge.

 

[RCinFLA]

The last two posts may have put the nail in the coffin for my 2p4s configuration. If I have separate 4s batteries, each with a 120 amp rated BMS, an 80 amp converter shouldn't be a problem, right? The BMS from OverKillSolar says it can do 120 amps charge/discharge.

Yes, generally. Just be careful about BMS current ratings. A lot of the Alib-BMS's will not support a continuous current of half their advertised rating.

 

[HRTKD]

I wish Victron would come out with a line of BMS for DIY batteries. A BMS that communicated with their other products through bluetooth would be nice.

 

[HRTKD]

A converter that was programmable, like many of the BMS and SCC products are, would be ideal. The converter would no longer have to be replaced when moving from lead acid batteries to LiFePO4. The only reason to upgrade the converter would be for more amps, which seems to be appropriate for LiFePO4.

 

[Sojourner1]

Why not just leave the converter you have in place (not hooked up) for the day you sell the trailer and just install a whole rig inverter/ charger (programmable) and you can take it with you when selling the trailer.
Just like Steve mentions.

 

[newbostonconst]

You are not going to be able to push 80 amps into a 280 ah battery for very long before it goes over voltage.

 

[HRTKD]

Why not just leave the converter you have in place (not hooked up) for the day you sell the trailer and just install a whole rig inverter/ charger (programmable) and you can take it with you when selling the trailer.
Just like Steve mentions.

It's looking like there is something wrong with my existing converter. It's stuck supplying 14.7 volts, even though the battery should be fully charged. It did this for more than 12 hours and the tech support at IOTA Engineering says it isn't supposed to do that. Last year when I had my trailer connected to shore power for almost 30 days while repairing delamination the converter came close to cooking the batteries. The electrolyte level was below the top of the plates and I know the level was well above the plates the prior month.

 

[Sojourner1]

You are not going to be able to push 80 amps into a 280 ah battery for very long before it goes over voltage.

80a /280ah= .28c

He'll have 560ah @12v
80/560=.14c

At times I've charged at 100a/500ah = .2c never been an issue.

 

[Forbisher]

80A into a 0.5C capable 280Ah LFP is no big deal if the BMS can handle it and has the correct HVD

This thread is Charge Amps for LiFePO4?

You are not going to be able to push 80 amps into a 280 ah battery for very long before it goes over voltage.

I will have the EVE 3.2v 280Ah cells, either in a 2p4s (single BMS) or a 4s2p (one BMS for each battery).

 

[HRTKD]

MBR, you are correct. I'm asking about charging a 560 Ah LiFePO4 battery bank. I wasn't sure what was acceptable. Based on most feedback, 80 amps is just fine, even if it was just one battery in the bank that was online.