Bluetti AC300, DIY Battery, Charging AMPS, Solarx 4k firmware

Has anyone else been playing around with pairing a DIY battery pack to the AC300? I've got a 16S LiFePO4 build using Fogstar cells and I'm trying to nail down the ideal charging current limits before I commit to anything permanent on my cabin setup.

The main thing I'm wrestling with is whether the Solarx 4k firmware update actually changes how the AC300 negotiates charge amps with an external pack — I've seen conflicting info on various threads and can't find anything definitive. Does it cap differently post-update?

A few specific questions if anyone has experience:

• What charge amp limit are you running with a DIY pack on the AC300 input?
• Has the firmware update affected your BMS communication at all, or is it purely inverter-side changes?
• Is anyone using a Victron SmartShunt alongside this to get accurate SOC rather than relying on Bluetti's own readings?

My BMS is a JK unit, which has been rock solid so far on my boat build, so I'm fairly confident on that side. Just not sure if the AC300's behaviour changes meaningfully with the newer firmware when it comes to respecting external battery parameters.

Would love to hear from anyone who's actually tested charging amps before and after the update rather than just going off specs. Real-world numbers would be genuinely useful here.

@Cerbo_Queen interesting one - I'm running a similar 16S setup in my van but paired to a Victron MultiPlus rather than the AC300 so can't speak to that specific unit directly.

What BMS are you using? I found with my Fogstar cells the BMS charge current limit matters more than what the charger thinks it's doing - had a mismatch early on where my charger was set to 40A but the BMS was silently throttling to 25A and I couldn't work out why charging was taking forever.

Also worth checking - are you planning to set the charge voltage at the AC300 end or relying entirely on the BMS to cut off? I've seen people get caught out expecting the BMS to handle everything and the AC300 behaving oddly when it gets hard-disconnected mid-charge.

What capacity pack are you building?

@Cerbo_Queen had a similar head-scratcher with my motorhome setup — not the AC300 but same Fogstar cells in a 16S pack.

Key thing I found: don't just trust the BMS max current rating, actually derate it by about 20-25% for continuous charging. Cells thank you for it long-term.

What BMS are you running? If it's a Daly or JK you can cap the charge current directly in the app which gives you a decent safety net even if the AC300 decides to push harder than expected.

Also worth checking the AC300's charge profile — some units have a UPS mode that behaves differently to normal charging. Could catch you out with DIY packs.

Really curious about this one actually — I've been toying with the idea of pairing a DIY LiFePO4 build to my narrowboat's AC300 for a while now.

@Cerbo_Queen did you manage to sort the charge current limits in the end? I've read conflicting things about whether the AC300's battery expansion port plays nicely with DIY packs or whether it expects specific communication from a "proper" Bluetti B300 module.

Also wondering — what BMS are you running with those Fogstar cells? I've seen people use a Daly or JK BMS to essentially "hide" the pack from the AC300 and let the BMS handle cutoffs rather than relying on the inverter's settings. Is that roughly your approach?

Would love to know what charge rate you're landing on relative to your cell capacity — C/5 seems the safe bet but curious what others are actually running in practice.

@Cerbo_Queen the AC300's B300 expansion port uses a proprietary comms protocol, so your DIY pack won't have proper BMS handshaking with it — worth being aware of that upfront.

For charging current on a 16S LiFePO4, the safe ceiling is typically 0.5C for longevity, though Fogstar cells can handle more short-term. The critical bit people miss is ensuring your BMS charge current limit is set below whatever the AC300 is pushing — the AC300 won't back off gracefully without native comms.

@LindaClark90 narrowboat installs are actually ideal for this kind of DIY pairing since you've got more physical space to mount a proper external BMS with solid current ratings.

What BMS are you running @Cerbo_Queen? That'll determine whether you can set a hard current ceiling reliably.