[RE-wrenches] How much Li battery?

[larry at starlightsolar.com]

Mark,

It is conceivable to use Li-ion without protection. To do so you must ALWAYS guarantee that you will not over charge, over current, over discharge or charge frozen cells…ever. This is a 0 tolerance rule. In a RE system, how can you be 100% sure? 

Modest rates is not the issue. It’s already difficult to overcurrent the GBS cells. Over voltage a cell just once and you can cause damage or possibly fire. Similarly, if you over discharge a cell just once you can damage it and may cause a future fire while charging it normally.

Cell balancers only reduce a small amount of current on a single cell that is becoming saturated. It won’t stop over voltage.

To my knowledge there is no safe way to have a Li battery system without BMS/EMS unless you are always in control of charge/discharge events. 

Scold…now that you know, yes, consider yourself scolded.

Larry Crutcher
Starlight Solar Power Systems



On May 26, 2020, at 12:24 PM, Mark Frye <markf at berkeleysolar.com> wrote:

Thanks Larry,

I am familiar with GBS cells. Would you scold me too badly if I told you my thought that with modest discharge rates and on the charge side, with cell balancers installed, and limiting the charge current to 15A, one doesn't really need the BMS for the 60AH pack? 

On 5/26/2020 10:47 AM, larry at starlightsolar.com <mailto:larry at starlightsolar.com> wrote:
> Hi Mark,
> 
> One of the many benefits of Li-ion is the ability to provide much higher continuous current than any comparable size LA battery.  
> 
> You don’t need more than 2kWh (40Ah) and want to know the smallest battery you can use. To directly answer your question: A 60AH Elite Power Solutions (GBS cells) battery bank @ 48V has about 3 kWh of energy storage. This is a tiny battery bank, just 20”L x 11”W and weighs only 80 pounds. However, it can provide a whopping 9kW (3C) continuous output and up to 31kW momentarily! Thats much more than you will likely ever use. 
> 
> Though hard to imagine, this will work with your inverter. We have built several 3kW inverter systems @ 12V with a single 100AH battery for starting heavy loads like air compressors and air conditioners. No problem. Whether this is the “best practice” will require more discussion. Let me know if you need more detailed help as we have many years experience with Li-ion battery systems.
> 
> Larry Crutcher
> Starlight Solar Power Systems
> 
> From: Mark Frye <markf at berkeleysolar.com <mailto:markf at berkeleysolar.com>>
> Subject: [RE-wrenches] How much Li battery?
> 
> In the way back machine I remember being at a product roll out meeting 
> for the brand new Outback FX inverter. I asked the engineer, what was 
> the smallest battery bank that could be connected to the inverter. He 
> said on the fly that probably you would want the battery to "weight" at 
> least as much as the inverter. I thought that was a great answer.
> 
> Fast forward, with a 4000w Outback Radian, AC coupled with 2000w of PV, 
> what is the smallest Li battery I would want for stable operation?
> 
> My loads don't really need more than 40ah of capacity.
> 
> What is the latest best practice on this?
> 
> Thanks.
> 

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