Charging 6V 12Ah Gel Battery: Comparing 5.7V 800mA vs 7V 800mA Chargers, Safety & Efficiency

Hello. Is it possible to charge a 6V 12Ah gel battery with a 5.7V 800mA or 7V 800mA charger? If so, which one will be better? Is it safe for the battery? I am asking because I have such chargers at home.
The battery has the following information:
Rechargeable gel battery
Charging voltage characteristics:
7.20V - 7.50V cyclic operation
Standby 6.75V - 6.90V
Maximum charging current 3.6A

The 7V charger is the closest one, but it would be good if you were charging with a 7.4V power supply.

Simply a 7V charger will not charge this battery fully and that's it.

From what I know, batteries do not fully charge, only preferably 80% to extend its life. So the 7V power supply is too weak and I was afraid it might be too strong. At alledrogo they sell chargers for gel batteries and described e.g. output 6v 800mA so according to the description on the battery (that you should charge with 7.20-7.50V) is also not suitable. My someone has 12aH and charges with a value of 0.1A from the amount of aH (I found this information on the forum but I don't have the knowledge to confirm it), so I conclude that the best power supply would have 7.35V 1.2A but probably 7V 800mA will also charge only that longer?

The second case the 7V 800mA charger is original from an old screwdriver that had 10.8v 1.3aH li-ion batteries. So when I saw that, I was stupid at all.

Theoretically, it can be charged with a 7 V charger. The gel battery from the theme (has three cells) and is a lead battery, so it is different from those for which the charger was intended. It can be easily assumed that the battery charger has a final charging voltage of 12.5 V (also three cells). It's too much for the gel battery from the subject. Personally, in order to check if it could be charged, I would connect the charger to the battery and use a multimeter to check if the charging current does not exceed the nominal current of the charger (800 mA). If the current does not overload the charger, I would periodically check the voltage increase on the battery being charged (with the charger connected and working). After reaching a voltage of 6.9 - 7.20 V, the battery can be approximately considered to be fully charged. Depending on how the battery was discharged and what is its actual capacity (new or used battery?), The charging process with this charger may vary from a few minutes to even a dozen or so hours. The final conclusion - it can be recharged but you have to make sure that something doesn't burn or when you finish charging. The battery, when it is charged and is being charged further "by force", loses its efficiency because the electrolyte density and volume change by accelerated decomposition of water into hydrogen and oxygen, which gases are irretrievably released into the atmosphere.
You can try this second 5.7 volt charger. Often, the chargers' voltages are given as conventionally accepted for various types of batteries and do not reflect the real values that the charger achieves, i.e. it should be experimentally determined (measurements by means of a current and voltage meter) whether charging will begin at all and if so, as before or not charger current and at the end of charging does not overcharge the battery or vice versa can not be able to fully charge?

As I wrote earlier, the charger is 7V 800aH originally from 10.8v 1.3aH Li-ion batteries. After charging the battery from the screwdriver, the green LED was on and the red LED was off.

The diodes signaling the charger were working with original lithium batteries - now there is nothing to count on. Often, chargers use the ?V function, which detects a slight voltage drop at the end of the charging period, and lead batteries show quite large voltage fluctuations throughout the entire charging process, which can confuse the charger and turn off sooner than needed. Let's assume that both chargers will charge (the ?V function will not disturb), so that you do not overcharge the battery, you need to assess for yourself whether it is already charged or not. Preferably by measuring the voltage quite often with a meter, details as I mentioned in post No. 5. However, there is a risk that we will miss the moment of charging the battery because it is not known to what extent the discharge is connected and how much time the charger needs at its performance to charge it. If we do not stop charging in time it will be to the detriment of the battery.
This charger with lower voltage gives some hope, but the charging process must be carried out experimentally and conclusions should be drawn. Try and write what came out. I will add that chargers may react to an uncertain connection with a battery by lighting a diode indicating the end of charging. For this reason, we measure the current at the beginning only to assess whether the charger is charging too high current and then connect the cables securely and only make the voltage measurements until the battery reaches the final charging voltage. If it reaches 6.9 V, we start to observe the voltage more often. If it exceeds the value of 7.2 V, we strongly increase vigilance and if it continues to increase, the final limit is 7.5 V. Personally, I would stop charging at 7.25 V with the conviction that the charger, unfortunately, does not turn off automatically and you need to finish charging yourself.

One more question: what current should one charge two 6V 12Ah connected in series which have 12V 12Ah after short circuit? I mean voltage how many volts and how many amps? Because I have two and I could load them simultaneously.

dziamber007 wrote:

what current should be charged with two 6V 12Ah connected in series which have 12V 12Ah after short circuit

The principle adopted by convention is: charging current = Q / 10 - what is meant by the capacity of the battery being charged divided by 10, i.e. in this case we have 12 Ah / 10 = 1.2 A. Everything is clear if the charger has the ability to set the charging current. If it gives less - charging will take longer. If the charging current is greater than 1.2 A, then the charging will be shorter, but unfortunately it affects the battery life and especially when we exaggerate with the charging current because the high current density heats the active mass of the battery plates locally and can also cause strong point gassing the active mass peels off the surface of the lead mesh that forms a current drain from the battery plate.
When it comes to the final charging voltage, it is obvious that by connecting two 6 V batteries in series we double the values applicable for a single six-volt battery and we get:
- voltage close to the end of charging 6.9 V * 2 = 13.8 V (from this moment we control the voltage increase on the battery pack more often)
- 7.2 V * 2 = 14.4 V is the final charging voltage. At this voltage, the charging current should already be much smaller and approach zero. If the charger itself does not reduce the current, then the voltage will continue to increase and depending on the parameters of the charger it can reach 18 to even 24 V but this means that the battery has already been partially or completely destroyed by too long and too intensive charging. In cars, damage to the voltage regulator can lead to an increase in voltage up to 60 V and sometimes above 100 V (alternators are very efficient), unless the engine driver is damaged.
Therefore:
It is better to charge with a current less than or equal to Q / 10
Do not allow the battery to overheat during charging.
Do not overcharge the cells, i.e. turn off the charging in time or use the charger with efficient automation and designed only for this type of battery.
The final charging voltage can vary depending on the condition of the battery by a few tenths of a volt up or down. Thus, the values given above are helpful but do not specify conditions for all lead-type batteries.
If you want to expand the topic of charging two batteries in series or in parallel.

Usually the batteries are charged with q/10, where q is the battery capacity.
Example: a 12Ah battery is charged with 1.2A for 10h, or with 2.4A for 5h...and so on.
So, when charging a battery not only the voltage of the charger matters but its current too.