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Craig Myatt avatar image
Craig Myatt asked

BYD 2.5KWh Battery and Multiplus II, off grid.

Hi, Victron recommends (https://www.victronenergy.com/live/battery_compatibility:byd_b-box#known_issues) having at least 2 x 2.5KW BYD plus modules, for a 48/3000/35 Multiplus, when in off - grid configuration. I am hoping to use the Multiplus II (35A charge current), with only a 2KW solar PV as the charging current, via a Bluesolar 150/60 MPPT. so the MAX charge current, I estimate will be about 30A, or about 0.6C for a single 2.5KWh BYD module (50Ah), which is what I plan to use...rather than 2 x 2.5KWh modules. Could anyone comment on whether that makes sense? I don't wish to use 2 x 2.5KWh modules, because we overspec'd the Multiplus, for later expansion, at which point there will be 2 x 2.5KWh modules, and 3KW solar.

Multiplus-IIBYD
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3 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi Craig,

When it comes to maintaining warranty, there isn't really a choice, you must follow BYD's minimum sizing specification.

One reason for this is the instantaneous current delivery limit, the maximum amps that the BYD can deliver before self protection mode. The multiplus has a large bank of capacitors, if you connect the multi to a battery for the first time, it is common to see a large spark.

I don't have the exact figures, but I have seen the behaviour. What can occur is that the 3k multi might draw 180A for 0.1s from the battery to charge the capacitors on connection. The BYD will quickly disconnect itself internally over 100A. So there is a battery overload / inverter undervoltage cycle that occurs. The internal BYD contactor is not designed to repeatedly break 100A of 51VDC current and it will eventually burn it out.

Now there are ways to work around this problem. You can pre-charge the multi's capacitors with another regulated DC supply (such as an MPPT), or an AC supply such as the grid or generator before you connect the battery.

But as neither of these can be depended upon in an off-grid situation, it is not approved.

There are other reasons also, but this is just one example. It's not just to sell more batteries :)

As my name tagged with Victron, I have to say please follow the manufacturers recommendations for a good, reliable and well tested experience. I have this exact configuration myself at home.

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Craig Myatt avatar image Craig Myatt commented ·

Thanks guy, i have now been asked to put 2 x 2.5KWh units in, so how a single 2.5KWh unit will perform is a moot point, however, that info about the capacitors is interesting, might be worth applying that when connecting the batteries. Are BYD considered durable for the 10 year warranty period? Is there a good history of durable use in Australia?

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Guy Stewart (Victron Community Manager) avatar image Guy Stewart (Victron Community Manager) ♦♦ Craig Myatt commented ·

BYD are one of the few LiFePO4 battery manufacturers that have over 10 years of in-field experience with their product. But much of that was in the more controlled environment of telecom market. BYD buses use a similar pack and have got a MASSIVE amount of real world data behind them.

So they would seem to be among the best you'll get at the moment. This is also backed by the lowest fade in the Battery Test Centres lab results - http://batterytestcentre.com.au/wp-content/uploads/2017/07/Battery-Testing-Report-September-2018.pdf

The product has not been in Australia for 10 years, and our market has already revealed some weaknesses, BYD have been quick to accept and work to address.

Thumbs up from me.

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hanco avatar image hanco commented ·

Hi @Guy Stewart (Victron Community Manager) and @BYD_service,

I want to combine a 3kva MP-II with a single B-BOX LV Flex 5.0 battery, configured as a basic UPS system (single phase) to cover load shedding in South Africa. The sizing requirements for this Inverter/BYD combo according to the victron/byd manual is
1702037789619.png


Does load shedding (loss of grid in UPS system) count as On-Grid or Off-grid? Will a single B-BOX LV Flex 5.0 battery be sufficient in a UPS configuration ?

Thanks.


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byd-service avatar image byd-service hanco commented ·
It would be equivalent to off-grid during a load shedding event.
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boekel avatar image
boekel answered ·

It's not a Victron recommendation, but from the battery manufacturer. These batteries are pretty 'weak' compared to other lithium batteries, and when they get below room temperature they accept even less power.

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Craig Myatt avatar image Craig Myatt commented ·

Does "weak" infer a "weak offering", in terms of performance? That is, the BMS is unable to manage off-grid charging, to the point of overcharging? Or "weak" meaning the ability to accept charge/discharge is weak? the BYD specs suggest a 1C charge & discharge, although the tests on the specsheet refer to 0.5C (0.5 x the Ah rating of 50Ah). I know Lithium batteries prefer not to be (ie last longer when not kept there) at full charge...I wonder if there is a way to maintain lithium batteries at say 90% SOC when not being used (in this case frequently...it is a weekender), such that the batteries last longer. That is, the MPPT can be throttled at a certain SOC %age, depending on whether the batteries are "in use" at that time. I guess not...without serious hacking...

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boekel avatar image boekel ♦ Craig Myatt commented ·

'weak' in terms of power, a result of a design choice between capacity and power vs price, they make a battery primarily designed for 0,5C continuous power rating, which is perfect for many systems (storing solar plus backup).

For 95% of systems it doesn't make sense to be able to discharge in two hours (0,5C) or shorter, so that is a small market they don't serve with these batteries.

If you use a Venus device, this can limit the charging and discharging to the BMS limit, but discharging is only limited when you are connected to grid (ESS system), there is no other way to limit the discharge in an off-grid situation other than having an AC breaker on the output of the inverter with the correct current limit.

It might be that the cells inside the units can handle the current without problems, the limitation can also be in the amount of heat the (closed?) housing can dissipate.

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byd-service avatar image byd-service commented ·

Hello Boekel,

I understand this is a service chat and I shouldn´t be making sales pitches but I have to make a comment that as Guy says if the surge power is 180A this is close to 4C. This is well above what any off-the-shelf battery in the market. In general, most lithium batteries operate at 0.3C-0.5C but the BYD battery box is one of the few that can accept a nominal of up to 1C and surges of 2C. I refer again to the ARENA report that shows it was the one operating at the highest power of all batteries tested http://batterytestcentre.com.au/wp-content/uploads/2017/07/Battery-Testing-Report-September-2018.pdf

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boekel avatar image boekel ♦ byd-service commented ·

@BYD_service

Please correct me (or others) if you see errors in the answers.

I was under the (wrong?) impression these batteries have a 0,5C continuous rating, and this is fine / perfect for most solutions, I should have used a different term than 'weak' I guess, as I had to explain in the next post what I meant with that.

What is BYD's official continuous discharge rating? (charge is in the manual) I'll edit my posts with this information.

For high power setups there are batteries available with 1C continuous or even higher ratings. I have never found the need for such high outputs, but there are use cases that need it.

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byd-service avatar image byd-service boekel ♦ commented ·

@Boekel thank you for the clarification. The confusion is understandable as it power is conditional on a number of factors.


I´ll take the Battery box 2.5 as the reference. It allows a continuous charge of 1C (50A) and short surges (several seconds) of up to 2C (100A). However, we need to take into consideration that the charge power of the inverter fluctuates so in order to make sure the battery operates always below 1C, by default for most inverters the charge value is set to 0.7C (35A) (discharge is less restrictive so by default it´s 1C). There are inverters that are set to 0.85C and even 1C but as I say the most common value is 0.7C (like the case of Victron).


However, you are right that most off-grid applications we see have lower power than capacity requirements and these batteries are not intended for power applications.

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c2519028 avatar image
c2519028 answered ·

I have a similar config, but I started with 3 battery modules and have upgraded to 4 now.

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