Lithium (LiFePo) batteries in strings

HI;
I’m planning on setting-up a 600AH solar battery bank comprised of LiFePo 12V 100AH batteries, which seem to be the most common (and reasonably priced) type offered by China manufacturers. In order to have a 48v system, it appears that I would have to have 6 parallel strings of 4 batteries connected in series. Is this design feasible? I have seen some solar sites which indicate that max # of battery strings limited to 4 only. Would this set-up need any special balancing circuitry or maintenance?
Your expert advice would be appreciated. Thanks

One for you @Sparky.

Hi Chewface,

I definitely would not do what you have suggested. You do not setup a lithium battery bank the same way as lead-acid batteries. Most lithium batteries (depending on the BMS) cannot be used series to increase the voltage as you suggested and this could be dangerous.

Unlike lead-acid batteries which use 2V, 6V or 12V cells in series to get 48V, with lithium you simply purchase a 48V lithium battery module. These are common in Australia and the US and brands like PylonTech and BYD are Chinese so I would recommend these batteries.
Most lithium batteries systems are only designed to be used in parallel, not series.

What inverter are you planning on using? This will determine what lithium battery you can use.

Jason

At Most 2 parallel bank more than 2 ( can be a Fire Hazard ) not advisable hard to balanced the charge
Get a pro to design the sys Good luck

Regards Mel Solar Hybrid Solutions

Hi Mel, Martin, Jason;
Thks for your respective replies.

In the Philippines, esp in the provinces outside Manila, you can find only solar equipment ‘installers’ (however not easy to find an experienced one).
Therefore, solar customers (like me) must buy the solar gear (including the inverters) or allow the installer to buy it, but as you might expect, at a (much) higher marked-up price. Warranties are only from the manufacturers.

FYI, the electric utilities do NOT buy any excess power produced from solar set-ups and in some places, off-grid solar systems are not permitted. However, as long as I pay for a back-up grid tie, then I should be able to run off my solar system 95%+ of the time…

One installer who i have contacted recommended installing qty-2 5KW “Axpert MKV” inverters (Taiwan brand I think). However, he was totally unfamiliar with LiFePO batteries or their set-up, which is why I am doing the research myself. Also another system he installed is starting to fail after only 4 years, which I suspect is due to the lack battery balancing circuitry…

How much would it cost to have a ‘professional’ design my solar system?

Thanks

Hi Chewface,

The most affordable solar/battery inverters are Victron Energy using DC coupled solar controllers. Do you know how to use these?

Can you purchase BYD lithium batteries in the Philippines? These are modular batteries which work with the Victron inverters. Very safe and reliable with 10 year warranty.

If you cannot get Victron or BYD then you need to find ‘self managed’ lithium batteries. This means they have an internal BMS (battery management system) and do not need to be controlled by the inverter with a comms cable.

Axpert MKV inverters are ok but very cheap and you might have some problems with software. They will only work with self-managed 48V lithium battery in parallel.

Jason

Hi Jason;
Thks for your advice. I checked with BYD and they do not yet offer their HV Li B-Box battery in either Singapore or the Phils. Just to help me scope out the economic feasibility, how much would a BYD HV Li battery configured for 600AH capacity and 8KW output cost in Australia? A rough cost would be fine.

Also in researching the battery it raised questions about AC-coupled vs DC coupled designs. My impression from reading various websites is that an AC-coupled design is good for existing PV installation due to the flexibility and simple installation. However does AC-coupled design for a new PV system offer any advantages?

Lastly, the DC-AC inverters used with HV Li batteries seem to top-out at 5KW, which means I would need to install 2 inverters on the same battery bank? Any design limitations or best practice guidelines for doing this?

Appreciate your guidance and advice.

Hi Chewface,

Do not use the BYD HV batteries. Only the BYD LV (48V) batteries will work with the Victron inverter/chargers. The BYD B-Box are some of the lowest cost lithium LFP batteries available and are also modular so you can add more capacity if required.

I would recommend using a DC coupled system using the Victron Multiplus inverter together with the Victron MPPT solar charge controllers and BYD B-Box LV batteries.

Hope this helps,
Jason

Hi Jason;
Been doing more research on what’s available in the Philippines and there is a Victron distributor, with whom I’m currently getting a quotation. Unfortunately BYD 48V LFP batteries are not yet available, only Pylontech. Are you familiar with this brand?

Also wrestling with another design question, on which your expert opinion would be much appreciated.

I’m trying to design the system for maximum resilience, since solar components are not easily available, expensive to ship and take a long time to bring in.

My initial paper design is to feed an 8KW PV array into a 600AH 48V LFP battery bank and to run 2 separate 5KVA DC-AC inverters (output 230VAC) off of the same battery bank. Each inverter will be connected to a completely separate distribution panel. Each DP will feed separate household branch circuits.

My question is whether connecting 2 separate inverters to a single LFP battery bank is OK? Do I need to sync the inverters, even though they are connected to completely separate loads?.

Grateful for any expert advice.Thanks.

Hi @chewface

Pylontech batteries are a great choice actually. In my experience even better than BYD. Here is some info about the Pylontech batteries - https://www.victronenergy.com/live/battery_compatibility:pylontech_phantom

I don’t see any reason why two separate inverters running in parallel would be a problem, as long as you stated that each will be connected to separately isolated distribution boards. With Victron inverters you can also connect inverters in parallel no problem. Syncing inverters is only required when running split-phase or 3-phase loads.
You will need to use a common BVM to monitor the battery state of charge.