I get a lot of questions for support from installations where the battery has some form of issue, they dont but here is a guide to understanding solar batteries and how to use them.
Battery Capacity
Voltage is NOT a measure of battery capacity, but it is used as the primary source to give some form of percentage. BUT not all batteries have the same voltage at a certain capacity and this can cause all sorts of issues with battery management.
Battery power is AH and this is a variable, for example a battery rated at 100ah may be tested at 50amps draw, while the same battery may be rated at 160ah at a 30amp draw.
Battery voltage and range.
Most batteries nominal voltages are derived from lead acid batteries. however the voltages are often different:
Lead-Acid: 2V
Nickel-Cadmium (NiCd): 1.2V
Nickel-Metal Hydride (NiMH): 1.2V
Nickel-Zinc: 1.6V
Lithium-Ion (Li-ion): 3.7V (various types)
Lithium-Polymer (Li-Po): 3.7V (various types)
Lithium-Iron-Phosphate (LFP): 3.2V
Sodium-Ion: 3.2-3.5V
The voltage range of a lead-acid battery is typically 1.85V to 2.1V per cell, The voltage range of a LFP (Lithium-Iron-Phosphate) battery is typically 2.8V to 3.6V per cell.
Looking at the difference at 12v battery packs, we know that an LFP battery would consist of four cells connected in series, with a total voltage range of approximately 11.2V to 14.4V.
On the other hand a lead acid battery consists of six cells connected in series, with a total voltage range of approximately 11.1V to 12.6V.
The inverter hardware is usually set for a lead acid battery which of course differs from the other types of battery available.
Our Li-ion battery consists of a 7S configuration on our 24V systems. This has a range from 21v to 29.4v. A typical 12v Li-ion battery would consist of 4 cells, and you would consider 8 being correct for the 24v system but this would be incorrect due to the range. 24v-33.6v
Why?? This is rather simple the LA charge voltage max is 30v, the max voltage of a 8S would only charge the cells to 3.75v (90%), most BMS would not balance until 4.1 volts and this can lead to pack problems.
In the same LFP home battery storage comes in two flavors, a 16s and 15s as the range is within that of 48V Each one will give you a different voltage as to the state of charge of the battery and LFP is fairly hard to determine the SOC due to the flat voltage of the cells. For example between 70% and 50% is 0.64v, 60% to 40% is 0.8v.
Battery turns off early – battery drops to zero.
This may sound odd, or its started to make sense as you now know about voltage range, but there is another condition that takes place that alters the capacity of the battery. This is usually controlled by the BMS but the works also in tandem with the inverter. Cell imbalance.
A BMS will have OVP ( high volt cell) and UVP (low Volt cell) and this can be minor and grow over time into a bigger issue, this is typically because the settings are wrong.
Lets look at the issue from the cell point.
A cell resistance can alter the rate of charge and discharge. this can cause the cell to have a different start or end point when it comes to charging. lets use voltage to show how the issue happens.
a cell discharges slower than the other cells, when it comes to charging, the cell has a higher voltage triggering OVP. the other cells would be under charged.
The user sets a low voltage and high current charge.
When the voltage is low, this can often be below the voltage level that the BMS will balance the battery pack. higher currents do not allow the batteries to balance or push up the voltage causing the pack to shut off before its fully charged.
if we use the analogy that if there is 2% difference in the cells, and each time there was a full cycle the battery with 2% more charge would complete 2% faster. as the cells discharge, the 2% becomes slightly bigger well will say 2.5% and this cycle repeats making the cells grow wider apart.
It is important to BALANCE your pack cells. and also parallel packs or serial packs.
Balancing your battery.
When you have your battery packs arranged as multiple batteries it is important to reach the voltage point at which the balancer can function.
When setting the inverters charge voltage this should be above the balance threshold.
Float or absorption is a important factor. To give you some insight, voltage and amps is two parts of the measure of power, Amps or current is actually the electrons and the number of electrons is your battery capacity.
To add electrons to the battery, you have to have a stable voltage, the cells voltage goes up as the cell is charged, however a high current can push up voltage without the current ( electrons) flowing. This would be where the electrons cannot be absorbed into the chemicals.
Think of it like having a small cup and trying to fill this with a tap turned on full blast, its just not going to happen.
As a battery charges the current level drops off. however if you have out of balance cells the pack may stop charging when one cell reaches OVP. therefore the balancer power has to match the flow in order to charge the lower cell and discharge the higher voltage cell.
This balance can be 0.5 amp or 1 amp and could be 4 amps and is active or passive. Passive balance burns off the power, where as active balance transfers the higher battery to the lower one.
Most people will run their battery between 20% and 80%, this extends the life of the battery if the draw current is also kept low. The current used to charge the battery should be as low as possible, but this can be hard to do when solar power is variable and unpredictable.
if you have a DIY battery, and it is easy to remove the cells, you may wish to remove the cells and connect them in parallel to charge them at 3.65v. This can take a few hours to undertake and then put the battery back together. for most, setting out the battery as a 4s 12v battery may work as it is more likely that you will have a 12v battery charger.
Fit a active battery balancer.
By far, a active battery balancer will assist in balancing the battery pack, this can take some time if the battery is out of balance, and may require you to charge the battery cells to create a even battery. or undertaking a full discharge and then recharging the battery to full voltage.
High voltage ( equalization) charging.
Some inverters will allow a equalization charge, this was used for balancing lead acid batteries and maintenance, typically this is 30volts on 24v systems. This can also be absorption on LFP or other types of batteries.
It is worth mentioning that 3.65v is a charge voltage not to be confused with charged voltage, that is normally 3.45v-3.52v per cell (51.75v -S15 55.2v -S16)
Holding the charged voltage allows for current to flow into the battery cells and charge any low cell, provided the current is low enough to not push other cells voltage high.
High voltage can be a problem if you do not monitor cells, as a low voltage cell and a high voltage cell would appear to be fine. lets say 7.3v for two cells could be 3.3v and 4v, this is a issue with serial battery charging (normal pack charging). But if the cells are in parallel, the voltage remains level across all cells.
Low capacity cell,
A low capacity cell and one that is out of balance can appear to be the same. If you were to parallel charge both cells and then disconnect them a damaged cell would appear to lower the voltage over a period of time as it settles.
Things to look for and prevention.
When charging review the individual cell voltages.
Fully charge the cells every few weeks
When at lower voltages, review battery cell voltage under high load ( instant) and look for voltage drops.
Checking packs should be undertaken without packs in parallel.
Update BMS firmware.
Update inverter firmware.
There is a lot to think about when working with batteries and their maintenance. mostly the problems appear much later and are from a very minor issue which gets worse over time.
if you do not know what your doing, its best not to mess about with any settings or take a battery apart as it can kill you and others.
Services from RenewSolar.
Send in repairs.
Charger hire.
Active balancer fitment.
BMS upgrades and installation.
Battery Building.
Cell replacement.
Warranty claims and reports.
Capacity testing.
Cell replacement.
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