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Old 10-20-2019, 05:16 PM   #21
john61ct
 
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Default Re: Building a LiFePO4 system

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Originally Posted by Midwestdrifter View Post
The master SS FET on drop-ins can and do overheat.
Drop-ins have all kinds of failure modes inaccessible to the owner.

In effect you are assuming a great warranty, that company will honor it, and last long enough to do so.
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Old 10-20-2019, 05:59 PM   #22
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Default Re: Building a LiFePO4 system

That's debatable whether quality or temperature can lead to earlier demise. Temperature can take out a component within seconds. Been a designer of man space flight rated systems. These systems must have the highest degree of mission success. Temperature is extremely critical.

Don't worry, I will purchase the cells from one of the companies you mentioned.

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No, the biggest factor is the quality of the cells internal to the battery.
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Old 10-20-2019, 06:06 PM   #23
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Default Re: Building a LiFePO4 system

Here is the DC-DC smart charger. I believe its fully programmable.

https://www.victronenergy.com/blog/2...dc-dc-charger/

Here are the victron battery protect unit.

https://www.victronenergy.com/batter...attery-protect

You may need another piece of hardware to network these together.

You could also use one of the generic BMS sold from various asian sources, and use its relay outputs to control the Solid state relays.
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Old 10-20-2019, 06:22 PM   #24
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Default Re: Building a LiFePO4 system

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Originally Posted by calbiker View Post
That's debatable whether quality or temperature can lead to earlier demise. Temperature can take out a component within seconds. Been a designer of man space flight rated systems. These systems must have the highest degree of mission success. Temperature is extremely critical.

Don't worry, I will purchase the cells from one of the companies you mentioned.
I'm warming up to your plan, especially if you purchase known good cells, hopefully from a distributor, as counterfeits are very common. I am also a big fan of LiFePO4s, and have several packs, including DIY that I use, but am still uncomfortable with them below my bed. I would never have other types of Lithium cells there. Used Tesla batteries really concern me.

I've worked in Avionics, a little different from manned space flight rated systems, but also with a keen interest in safety. My aversion to DIY systems stems from the fact that most that attempt it do not have an expertise in designing such systems. They don't know what they don't know. Also, I have designed systems where a flaw was not detected until there were thousands out there for years. I'm a pretty good engineer, but not perfect! In this regard, avionics have to be more reliable as the chance of catastrophic failure must be minuscule. There are so many more out there that even a low probability of failure X Units X Hours gets to be a big number.

I've also had big problems with counterfeits and failures of components that were even purchased through proper channels. Most of the new component purchases I have made from other than distributors have been fakes. I only buy from official distributors now.

The numbers I provided were for the entire system, wiring and all, so I think the internal resistance of the batteries has to be significantly lower than 20 milliohms. I am also a fan of parallel batteries.

Regards,

Mark
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Old 10-20-2019, 06:44 PM   #25
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Default Re: Building a LiFePO4 system

I understand you. My rig is a Winnebago motor home. There are no kluge components in there. It's unlikely the smaller microwave will fit in that location, or look good after install.

In addition, the mw also serves as a convection oven. It gets use cooking pizza, etc. I'm pretty much locked with this unit.

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Originally Posted by Graphite Dave View Post
Still a $75.00 microwave is a lot less expensive than a new battery system. Very easy to eliminate the generator.
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Old 10-20-2019, 07:11 PM   #26
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Default Re: Building a LiFePO4 system

There is some interesting discussion in this thread. At first the OP thought he had a bad cell, but it ended up being a bad connection with the terminals.

https://www.expeditionportal.com/for...-cells.209203/

I was looking at some of the 30-80A all-in-one DIY BMS units. They are usually current limited by the FETs used for the Solid state relay/disconnect. With your electronics background, you could likely replace the supplied FET package with something like the victron solid state units, or even just use more/better FETs with a decent heatsink.
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Old 10-20-2019, 07:35 PM   #27
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Default Re: Building a LiFePO4 system

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Originally Posted by calbiker View Post
That's debatable whether quality or temperature can lead to earlier demise.
Well yes, crazy high temperatures are very life-shortening.

As long as a LFP bank is used within the human body comfort range, should not an issue worth going to too much trouble about.

_____
Note that charging at high rates, say over 0.4C is only safe (conducive to longevity) at very warm temps.

So, if fast charging is critical, might be worth setting up a system that pre-warms the cells to around 30C.

I still wouldn't go much over 1C though.

Once things get actually chilly, reduce to 0.2C or even lower, and never charge once anywhere near freezing, too high a rate at too low a temp can render the whole bank instant scrap.

Last edited by john61ct; 10-20-2019 at 07:43 PM.
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Old 10-20-2019, 07:39 PM   #28
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Default Re: Building a LiFePO4 system

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My aversion to DIY systems stems from the fact that most that attempt it do not have an expertise in designing such systems.
Sure but failure modes with LFP are not any more dangerous - I would argue much lower risk - than with lead banks.

I'm not specifically advocating for under the sleep platform, but when the living space is tiny, needs must, not really different from under a seating bench.
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Old 10-21-2019, 01:52 AM   #29
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Default Re: Building a LiFePO4 system

Quote:
So why link to Kisae when discussing Renogy?
The OP mentioned he was looking at Renology and asked for "any better B2B out there?". I like my Kisae, so I linked to it.
The new Victron one posted here also looks interesting and the App would be nice for 1st time setup, but I haven't seen a price yet, this is a little weird: "Whilst you might expect it to report current – the Orion-Tr Smart is not currently designed to measure it, although that is something to be worked on for future versions." and it's limited to 30 amps. I like having 50 because I hate idling or driving just for the purposes of charging batteries.
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Old 10-21-2019, 02:04 AM   #30
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Default Re: Building a LiFePO4 system

It's not that simple as just using the victron solid state switch. This switch as well as fets conduct current in just one direction. Two switches, or two sets of fets are required. One for charging and the other for discharging the battery.

This 300A BMS looks interesting. Almost half the board contains the fet switches.
https://www.aliexpress.com/item/4000...chweb201603_52

Quote:
Originally Posted by Midwestdrifter View Post
With your electronics background, you could likely replace the supplied FET package with something like the victron solid state units, or even just use more/better FETs with a decent heatsink.
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