Idiot's guide to a self build battery

[Kenny000666]

I notice you guys have built 15s packs. Is this so that you get as close voltage as the pylons?

As I understand it, the pylons are nominal 48v (I assume it would be a bit higher at full charge. 15s packs will 55v? Why can you not just plug 16s into the pylons?

[Oldgreybeard]

I notice you guys have built 15s packs. Is this so that you get as close voltage as the pylons?

As I understand it, the pylons are nominal 48v (I assume it would be a bit higher at full charge. 15s packs will 55v? Why can you not just plug 16s into the pylons?

15S of LiFePO4 tops out at an absolute maximum voltage of 54.75V for 3.65V per cell, but for best life the BMS needs to shut down the charge at around 3.5V to 3.6V per cell, so around 53V to 54V. My stack of Pylontechs tends to show a terminal voltage of around 52.5V when fully charged, so it looks as if charging shuts down at around 3.5V per cell, which seems a bit on the low side, but I'm not 100% sure how accurate the voltage reported by the inverter is.

[nowty]

Why can you not just plug 16s into the pylons?

Because Pylons are 15S, voltage mismatch, ok its possible to limit your charging to 54V (or 53.5V with the new "C" variants) but that would mean the 16S cells only get to 3.375V or 3.344V with the new "C" pylontechs.

I added an extra LFP cell to my Pylontechs to better match my Growatt 14S LMCO batts.
Something I'm personally happy with, but I don't recommend it for others if possible.
https://camelot-forum.co.uk/phpBB3/view ... =60#p16807

[Stinsy]

I notice you guys have built 15s packs. Is this so that you get as close voltage as the pylons?

As I understand it, the pylons are nominal 48v (I assume it would be a bit higher at full charge. 15s packs will 55v? Why can you not just plug 16s into the pylons?

The Pylontech batteries are 15s LiFePO4. Therefore if you are adding a homebrew battery in parallel 15s LiFePO4 is sensible and anything else (eg 16s) is “unlikely to be successful”.

[Caesium]

I notice you guys have built 15s packs. Is this so that you get as close voltage as the pylons?

As I understand it, the pylons are nominal 48v (I assume it would be a bit higher at full charge. 15s packs will 55v? Why can you not just plug 16s into the pylons?

Can only add my agreement to the replies above, voltage match and keeping it simple to get started. As mentioned in thread I have plans to "upgrade" mine to 16S, but only after all the Pylontechs are disconnected. I can do this because my inverter lets me set a charge voltage in lead acid mode - up to 60V I believe. If I wanted to get really clever I could probably pretend to be a Pylontech master battery via canbus and request a charge voltage of 56v (3.5v x 16).

There are many dozens of pages on voltages of LiFePO4s (all suggesting slightly different things, but generally that absolute-max voltage is 3.65v) but I'll have more than enough capacity once my second stack arrives (around 28kWh, of which I can only recharge 15kWh or so per night) so I'm not too interested in pushing the extremes of charge capacity. I'll be pretty happy getting them up to 3.5v (which is as near-as-damnit-full anyway, probably 90% SOC plus) and see where I go from there.

[Stinsy]

I notice you guys have built 15s packs. Is this so that you get as close voltage as the pylons?

As I understand it, the pylons are nominal 48v (I assume it would be a bit higher at full charge. 15s packs will 55v? Why can you not just plug 16s into the pylons?

Can only add my agreement to the replies above, voltage match and keeping it simple to get started. As mentioned in thread I have plans to "upgrade" mine to 16S, but only after all the Pylontechs are disconnected. I can do this because my inverter lets me set a charge voltage in lead acid mode - up to 60V I believe. If I wanted to get really clever I could probably pretend to be a Pylontech master battery via canbus and request a charge voltage of 56v (3.5v x 16).

There are many dozens of pages on voltages of LiFePO4s (all suggesting slightly different things, but generally that absolute-max voltage is 3.65v) but I'll have more than enough capacity once my second stack arrives (around 28kWh, of which I can only recharge 15kWh or so per night) so I'm not too interested in pushing the extremes of charge capacity. I'll be pretty happy getting them up to 3.5v (which is as near-as-damnit-full anyway, probably 90% SOC plus) and see where I go from there.

Would 16s give a higher charge rate?

[Caesium]

Would 16s give a higher charge rate?

The inverter should push in the same amount of amps in both cases - I believe its already doing all it can, about 70A I believe mine does.

Since the pack voltage is marginally higher this will translate into more watts going in (since P=VI).

Obviously the exact maths depend on the voltage of the pack as it charges, but lets say if a 15S pack spends most of its charging time at around 50V then thats 3500W.

A 16S pack by implication would be another 3.5V above that, so 53.5V * 70A would get me 3745W going in.

Not a massive difference but marginally higher charge rate in terms of power going in.

Sure someone will be along to correct the maths in a minute

Yeah here we go for the inverter specs:



70A max charge and it can do 60V, so 16S should be fine on this inverter. Happy days!

[Stinsy]

Can only add my agreement to the replies above, voltage match and keeping it simple to get started. As mentioned in thread I have plans to "upgrade" mine to 16S, but only after all the Pylontechs are disconnected. I can do this because my inverter lets me set a charge voltage in lead acid mode - up to 60V I believe. If I wanted to get really clever I could probably pretend to be a Pylontech master battery via canbus and request a charge voltage of 56v (3.5v x 16).

There are many dozens of pages on voltages of LiFePO4s (all suggesting slightly different things, but generally that absolute-max voltage is 3.65v) but I'll have more than enough capacity once my second stack arrives (around 28kWh, of which I can only recharge 15kWh or so per night) so I'm not too interested in pushing the extremes of charge capacity. I'll be pretty happy getting them up to 3.5v (which is as near-as-damnit-full anyway, probably 90% SOC plus) and see where I go from there.

I’ve been giving this some thought…

If you’re ditching the 15s Pylontechs, why go 16s and not 17s?

I believe your inverter’s charge voltage can be set to 42-60V, given that Victron recommend charging 15s Pylontechs at 52.4V then this extrapolates to c. 59.4V for a 17s setup.

[Caesium]

I’ve been giving this some thought…

If you’re ditching the 15s Pylontechs, why go 16s and not 17s?

I believe your inverter’s charge voltage can be set to 42-60V, given that Victron recommend charging 15s Pylontechs at 52.4V then this extrapolates to c. 59.4V for a 17s setup.

Don't tempt me! Other than my OCD taking over for the build (how do you make a 17S battery look neat without being lopsided?!) I expect it should work.

It hadn't really occurred to me at first. I would have to buy new active balancers as the ones I have only do 12-16S. The JK BMS will do up to 24S so thats a good start for future expansion.

I'm probably going to stop at two 16S packs for the time being, that'll be around 28kWh of DIY.

In a few years I may be in a position to get solar panels again after moving house - current house just isn't worth it, small SE facing roof with a chimney in the middle of it that would shade half the panels after midday. But at that point I might expand it to 3 or 4 stacks to try and move closer to self-sufficiency - could consider 17S then while buying more cells anyway. I used to have 4kWp on the original 45p FiT rate.. moving out of that house hurt!

[Oldgreybeard]

28kWh is a fair bit of storage. We have slightly over 21kWh and in practice we rarely ever use more than half of the available capacity. Most of the time the battery SoC swings between about 65% and 100%, but it's handy to have some extra capacity, both for power cuts and to give the batteries a gentler life. This plot shows the SoC of our system over the past week. The flat areas at 70% are the overnight off-peak slot, where the inverter is set to charge to 70% if needed and use grid power through the night.

Battery SoC.jpg (52.38 KiB) Viewed 972 times