Battery – for what purpose?

Reading time 5 minutes

Updated – November 28, 2023

What type of battery for what purpose? This question arises as the number of battery types increases.

The good old one Lead-acid battery as a six-cell starter battery for cars or motorcycles. Heavy, big, high maintenance. You had to check the acidity and level of the 37 percent sulfuric acid more often, top up with distilled water and make sure that acid splashes didn't burn your clothes.
Contrary to the assumption that the lead-acid battery would have to represent the heaviest caliber, at 200 Ah it weighs around 45 kg.

Then they came Gel batteries just right. Silica causes the sulfuric acid to gel. No refilling of distilled water, no acid splashes. Also ideal for cyclical loads. The perfect battery, with a long service life of 12 .. 20 years (20 °C), 6 .. 10 years (30 °C), or 3 .. 5 years (40 °C) or in charging cycles: 1,800 at 30 percent, 750 at 50 percent and 500 cycles at 80 percent discharge,
The gel battery in a 200 Ah version weighs in at a good 60 kg.

The AGM battery (Absorbent Glass Mat) has a glass fiber mat saturated with sulfuric acid, has a very low internal resistance and therefore a low self-discharge. It has very good cold starting capability and is preferred for applications with high power requirements. However, neither gel nor AGM batteries like deep discharge. The service life is 7 .. 10 years (20 °C), 4 years (30 °C), or 2 years (40 °C), or in charging cycles: 1,500 at 30 percent, 600 at 50 percent and 400 Cycles at 80 percent discharge.
A 200 Ah AGM battery weighs a moderate 51 kg.

Finally round Lithium (LiFePo4) batteries the range of offers. Compared to the above types, they save a good 70 % in weight with the same capacity, offer around four times the power when starting and around ten times the number of possible charging cycles. An 80 percent charge gives it a longer service life, but constantly charging it to 100 % stresses the battery and shortens its useful life. It reliably delivers 80 % of its capacity over a wide temperature range from 50 °C down to -20 °C. However, it requires at least 5 °C to charge. Some providers therefore offer batteries with built-in heating that ensures this minimum temperature. The lifespan in charging cycles: up to 5,000 at 50 percent discharge, 3,000 at 70 percent discharge and 2,500 cycles at 80 percent discharge.
A 200 Ah LiFePo4 battery is lightweight at around 14 kg.

A distinction must be made between LiFePo4 batteries with or without built-in BMS (Battery Management System). The BMS serves as a protective element against overcharging and deep discharging, and also ensures a balanced state of charge for all cells included (four cells at 12 V). If a BMS is not included, it must be purchased and connected separately.
There are also BMS without integrated Balancers (automatic cell balancing) is offered. In this case, this must also be ordered and connected individually.

In general, no battery likes high temperatures. She feels most comfortable at 20 °C.

The charging and discharging characteristics of each battery type are examined in more detail below. Intelligent chargers fully automatically determine the correct battery type and switch to the appropriate charging and maintenance mode. Chargers that can be manually switched to the appropriate battery type must be set accordingly. Chargers that can only charge a certain type must be replaced with a suitable device when the battery type is changed.

Terminology

In battery charging technology, a distinction is made between up to four different phases of the charging process. Chargers have fundamentally identical procedures and may even be able to recognize which type of battery is connected. One adaptive The control also takes the current power consumption into account when charging.

Bulk

Bulk refers to charging with constant current until the battery's absorption voltage is reached.

absorption

absorption is the continuation of the charging process with decreasing current but constant (absorption) voltage over a period of up to 4 hours.

float

float is a charge that lasts for 24 hours with a decreasing voltage of up to 14 V, which is followed by an absorption phase of one hour.

Storage

Storage follows the one hour absorption phase and maintains the voltage at 14 V for up to a week.

Lead-acid battery

First of all, a distinction must be made as to whether the lead-acid battery is used cyclically, as in vehicles, or, as in an emergency power supply, whether it runs in standby and is constantly charged to compensate for self-discharge.

The charging process for cyclically used lead-acid batteries is divided into three sections:

• Charging with constant current for up to 16 hours at 2.45 .. 2.5 V per cell
• Charging with constant voltage of 2.3 .. 2.35 V and continuously decreasing current for up to 8 hours
• Trickle charging with 2.275 V per cell and also decreasing current

The final discharge voltage, equivalent to deep discharge, is 1.8 V per cell, i.e. 1.8 x 6 = 10.8 V. Falling below this voltage can permanently damage the battery to the point of being unusable.

Gel-acid battery

The charging procedure is identical to the one above, but the charging voltages are different.

2.35 .. 2.4 V, for a 12 V battery corresponding to 14.1 .. 14.4 V represent the values for the maintenance and end-of-charge voltage.

While a lead-acid battery tolerates exceeding the end-of-charge voltage well, gel batteries take this very seriously and quickly stop working.

AGM battery

AGM batteries behave like gel batteries in terms of the charging/discharging process and voltage values, as well as exceeding the end-of-charge voltage.

Lithium/LiFePo4 battery

Lithium / LifePo4 batteries at 12 V consist of four cells rather than six. This means that the cell voltage is 3.2 V, the end-of-discharge voltage is 2.8 V and the charging voltage is 3.5 .. 3.6 V, ideally 3.55 V.

In battery networks, only identical batteries with identical capacities may be connected to one another.

BMS

As a BMS it will battery Mmanagement Ssystem designated. It serves to protect against deep discharge, such as overcharging, overvoltage, short circuit or charging attempts at temperatures below 0°C.

BMS contain either a built-in or connectable Bluetooth unit through which a connection can be established to the corresponding app on a cell phone or tablet and all relevant measurement data can be displayed and sometimes also configured.

However, the display should be interpreted with a certain degree of caution, because the capacity data displayed are calculated, not measured values. Since the voltage curve of a LiFePo4 battery is almost linear between 25 and 95% and only has a voltage difference of around 0.05V across this 70% charge state difference.

The BMS continuously draws a small amount of current from the battery, which is why there is a risk that the battery will become discharged over time. Therefore, when not in use, it is recommended that the battery be fully recharged after six months of storage at the latest.

Charger does not charge

If the BMS has switched off due to undervoltage, the charging function via a connected charger may also be deactivated, as some chargers require a minimum voltage to go into charging mode.
The easiest way to reactivate the charging option is to apply a voltage of greater than 14 V, such as via the alternator. So: start the engine, the alternator supplies around 14.5 V and thus deactivates the BMS shutdown. The engine can then be switched off again and the connected charger continues charging as usual.

Balancers

Batteries consist of several individual cells. These are connected in series and add up to the nominal battery voltage. One would assume that all cells are charged and discharged equally. Since this is not the case, you would have to periodically connect all battery cells in parallel, wait for a while until the cell voltages have equalized and then rebuild the serial connection.

An active balancer can carry out this process automatically, but without transferring the cells from serial to parallel connection.
The balancer is connected to each plus and minus pole of the individual cells of a battery, measures the respective cell voltage and then automatically decides to shift energy from a fuller cell to a weaker one.

A balancer complements the BMS with this balancing function, but does not replace it!