Hi all
Gelato 24v 2.56 kWh LFP battery.
The booklet I got with it and the spec sheet I found online state “standard charge voltage 28.8v”
Specs which I found on Solar Warehouse SA 2.56kWh Gelato Lithium Wall-Mounted Battery 100AH 24V | Solar Warehouse SA
Say
Bulk Voltage : 27.6V-28.4V
Float Voltage : 26.4V-27.2v.
So what should the values actually be, what voltages do I put in to my Victron MPPT?
If it is is LFP, it is almost certainly 8 cells in series, so 28.8V would be 3.6V per cell. Which is acceptable, but on the high side.
28.4V is 3.55V per cell. That sounds way better to me.
26.4V is probably for storage, and way too low (3.3V per cell) for daily use.
I would use 28.4V absorb, 27.2 float.
This battery have no bluetooth or other coms to the BMS to see cell voltages, so I’m just scared the cells won’t properly balance.
It’s used off-grid with a 100/30 MPPT and 600W PV, we’ll be adding a Smart Shunt later for better monitoring via bluetooth, just waiting for stock.
I would be alright with 3.6V per cell, if the absorb time is really kept short, and the rest of the time it is held at a lower voltage. In the application (which I assume is DC), the risk of accidentally overcharging is minimal. There is no grid interaction.
Ultimately only the manufacturer knows the real answer, but I imagine that it is designed to be a “drop in replacement”, and as a result they have to say “standard 28.8V” because that’s what a lead acid battery charger will do. I cannot imagine that it needs more than 3.55V to balance though. It’s a guess, but I would think an educated one.
If it is designed as a drop-in replacement, it also has to work in systems that don’t have multistage charging, and just run a constant 27.6V float. Which is quite likely why the lower end of “bulk” (aka absorb) is set to 27.6V.
Thanks, I’ll do some more research to see if I can get more details on the battery, but in the meanwhile will use your values.
Yes it’s a DC only system, the only charge source being the MPPT,
How would the typical inverter manage the charging cycle? I assume that as long as there’s PV power they will be kept at the float voltage…
But what triggers an absorb charge??
It’s one of those “not an exact science” things. Most of them pretty much go to the higher voltage, holds it there for a predefined amount of time, and then drop to the lower voltage. And that is that, nothing more fancy is done at all.
Victron inverters use a kind of adaptive charging, where it times how long it took to get to the absorption voltage, and then the amount it stays there is proportional to that (I think it is 5 times the time it took to get there). The idea is that if the bulk stage is really short, then the battery is probably already quite full, and absorbing too long is detrimental to the battery. I am not aware of any other inverter/chargers that use this method. Note also that this method is for LEAD ACID batteries. Not for Lithium.
If the battery voltage drops below a certain point (Something like 1.2V below the float voltage for lead acid, slightly less for lithium), the inverter/charger triggers “rebulk”, which means it goes back into bulk, absorb, and then float. The time it stays in absorb is again dependent on how long it took to get there. Again, not sure how others do it. Everyone has their own little thing that they do.
Then, Victron inverter/chargers also do a periodic absorption cycle, configurable. It won’t stay in float forever (although many others do exactly that… and it is fine for most cases).
Victron inverter/chargers also have a “storage” state. After 24 hours of floating, with no discharges, it goes to a lower non-gassing voltage (again, this is for lead acid). And in the same manner, an absorption cycle triggers every few days.
As I said, not an exact science. Everyone does it a little differently. Most simply do bulk → Absorb → Float, with a hardcoded time for absorb.
Lithium batteries don’t really need absorb, but it can be beneficial to have a similar two-step process. The higher voltage is high enough so that the balancer can operate well (ie, it should be high enough to push cells above 3.45V, possibly even 3.5V). After some time, it then drops to the lower voltage, where your cells level out around 3.4V to 3.45V, which is better for the longevity of the battery, and also switches off the balancer. For today.
Can I just also say… seriously… Gelato? Ice Cream in Italian? Who thinks of these things?! 
When I saw the name I got hungry.
This is what I’m seeing on my SMA inverter. They have this boost charge which is much like a full charge but is triggered on low battery voltage. This pretty much happens every day even if only discharging to 50% SOC.
I enquired with Trojan the battery manufacturer about a boost charge and the response I received was:
"If your battery bank is 48V, then your total capacity is 225AH at 48V. Maximum charge current is 13% of 225 = 30 amps. At 30 amps initial charge rate, the estimated absorption time period would be 3 hours. Charge voltage is 59.3V. "
PS: I don’t fully understand this…
Good day
Our 2.5kwh has a built-in BMS to protect the cells, and it has 8 cells inside. You can check the battery on the inverter, the voltage setting is attached as well.
For more information, kindly feel free to contact us directly on 068 835 1662 / 063 989 9999. We directly deal with the importing and distributing of this product alongside Solar Warehouse SA.