Why can not the car battery be used in the UPS?
Background
At the time when I first tried to replace the old 7Ah battery in the UPS with an old car battery with a nominal capacity of 65Ah, I still did not know why this should not be done, and how it could harm the health of the battery, the UPS itself and people living in one indoors with him.
Refinement of the uninterruptible system did not take much time, but the profit was immediately noticeable. The steady load in the form of a home "server" lasted about twenty hours without external power, although before 10 minutes - this was the limit, which was enough only for the correct completion of work. No longer shutdowns during the operation of this modification were noticed, and an Internet connection using GPON technology allowed the server to remain on the network even with large-scale power outages.
But it was a long time ago. A year ago, I accidentally came across an ad for the sale of several used UPS APC 3000 for ridiculous money, 4000 rubles apiece, without batteries, but working. After thinking a little, I decided what to take, and two at once, though by the time of purchase the price managed to rise to 5,000 rubles apiece, but that didn’t stop me, because in the store for the same money they offered only 1 kW options, and even then from all kinds of noname firms with not very flattering reviews and a modified sine.
The UPS refused to turn on without batteries, judging by information from the Internet, it needed eight 12-volt batteries, i.e. 96 volt battery, but the capacitors at the input of the batteries were 63 volts. It turned out that the cartridge has two parallel-connected chains of four batteries, 5Ah each. The result is a 48 volt and 10Ah battery. And here the fun began.
Battery Selection
It is time to buy batteries. The difference in price between specialized UPS batteries and ordinary car batteries was about two times with a comparable capacity. Why pay more? I decided to google it and found several sites selling battery for UPS, which almost backed up several reasons why they should pay more. In general, it sounds believable, but let's look at them in more detail.
So, the first significant difference is the different DC voltage in the car and at the source of autonomous power supply. For a car battery, the DC voltage is approximately equal to 14-14.2 V, and for a battery for uninterruptible power supply it is 13.5-13.8 V. The charge voltage for ordinary car and special UPSs is designed for different values. After you connect the car battery to the backup power supply system, the result will be seen as follows - the battery will constantly be undercharged. High internal resistance is present at the most charged battery, since a small current is consumed when working with the UPS. With discharged batteries, things are exactly the opposite. Ultimately, attaching a car battery can cause electrolyte to boil,
We look at the Wikipedia article on lead-acid batteries and we see that the emf of a charged battery is 2.11-2.17V, for 6 cans it turns out 12.66-13.02V. We look at the battery for the UPS and we see inscriptions about the recommended voltage values: in continuous charge mode 13.5-13.8V, in cyclic mode 14.4-15.0V. We look at a fully charged car battery, see 12.7V, start the engine, the voltage rises to 14.2. It turns out that 14.2V is not the voltage of the car battery, but the voltage by which the car generator charges it. But is there any battery charge scheme in the car? In general, this argument seemed to me untenable.
The second difference is the time phase of the work and the uniform release of electric current due to the plates that are built inside the battery. The average thickness of the electrode (plate) for a car battery is about 1-1.2 mm, and for specialized UPSs it is 2-2.5 mm. The movement of electrons occurs on a less thick surface. If you connect the car battery to an uninterruptible power supply, then the plates that are inside will quickly collapse due to the long functioning of the cycle.
If the car did not have an alarm and a radio, then one could probably believe that a car battery is not capable of delivering small or medium currents for a long time, but they are powered by the same battery. And this is not to mention that the car, in principle, can move for some time without a generator, only on the battery charge, and after that it will be enough just to charge the battery and it will continue to work. As for the thickness of the plates, it is difficult to say anything, except that some people come across nanotechnological glass inserts in the batteries from UPSs. Glass adds thickness to the plates and the weight of the battery, although it does not participate in chemical reactions.
And the third important difference is that hydrogen is released during battery charging. When the battery is installed under the hood of the car, the hydrogen quickly evaporates and does not pose any danger. Since the uninterruptible power supply is usually installed in a confined space, the gas will begin to accumulate, and the mixture of hydrogen and oxygen forms an explosive mixture that can detonate from any spark (even from the inclusion of light). The battery for the UPS is completely sealed; during operation, it does not emit hydrogen into the atmosphere, but recirculates in the battery space.
This argument immediately seemed suspicious to me, due to the fact that I had never seen a sealed battery in the UPS. If you look at the battery, you can see small openings for exhaust gases, unlike car batteries, they are closed with rubber caps and walled under plastic plugs, but not at all sealed. If you remove the plastic plugs and put the battery on charge, then some rubber caps will cheerfully fly away in an unknown direction. So the water still breaks down into oxygen and hydrogen, and a simple rubber cap will not make them turn back into water, and after a certain pressure, the gases will still come out. But okay, if after several years of operation of the car battery in a closed cabinet nothing exploded,
Car batteries have a diluted electrolyte, and since all processes are fast in a liquid medium, the service life of these batteries is much shorter than for specialized UPSs. Inside the battery for uninterruptible power supplies there is a spongy material that is saturated with electrolyte. And therefore, the self-charging current is small. And when the system switches to battery operation, the batteries for the UPS will work more.
Indeed, in a car battery, the electrolyte is in a liquid state, and in specialized batteries for home UPSs it is impregnated with porous material, and if you turn it over with open caps, nothing will pour out, this allows you to place it inside the UPS in any position, even up feet (although not recommended). I don’t know how this is connected with the self-discharge current, the complete electrolyte and the rate of chemical reactions, but most likely, nothing.
And do not forget that the car battery operates in harsh conditions, it requires high currents several times a day, it is accompanied by very low temperatures for several months a year and high for several months, in addition, it experiences vibration and shock loads during the car’s movement, and the generator charges it without any control, and it’s good if the owner monitors its condition.
Also, some people doubt that the UPS is able to charge a car battery, because it has a significantly larger capacity. But after increasing the capacity, we get an increase in the duration of battery life, it is strange to expect that subsequent charging will be carried out in the old time.
After reading several more articles about the dangers of using a car battery in everyday life, it became clear that nothing was clear. But, given the previous positive experience, it was decided to choose an option with a larger capacity, i.e. car batteries. For one UPS, the cheapest batteries from Tyumen Bear at 75Ah were selected, for the second BRAVO battery at 90Ah for about the same cost. And now, after almost a year of operation, I decided to try to measure the battery capacity in order to understand how bad everything is.
Measurement Results
| Parameter | Battery No. 1 | Battery No. 2 |
|---|---|---|
| Model | BRAVO 6CT-90VL | Tyumen Batbear 75 |
| Capacity max current | 90Ah, 760A | 75Ah, 610A |
| Cost at time of purchase | 2200 rub | 2400 rub |
| installation date | November 9, 2014 | November 11, 2014 |
| Ups | APC Smart-UPS 3000VA, 2700W, 230V, pure sine 50Hz + -3 Hz | |
| Load | gas boiler pump, floor heating pump, water well pump, freezer, refrigerator, lighting | lighting fridge |
| Charge-discharge cycles | 330+ | 10 |
| Calibrated | not | Yes |
| Date of control measurement | August 31, 2015 | September 1, 2015 |
| Control discharge | 4 hours 20 minutes, 37.22Ah | 9 hours, 55.7Ah |
| Voltage after discharge | 45.0V under load, 48.7V without load | 44.6V under load, 46.3V without load |
| Control charge | 9 hours, 37.32Ah | 14 hours, 52.28Ah |
| Voltage after charge | 55.4V, plus or minus 0.02V on each battery | |
| Electrolyte level | Visually unchanged, level above the plates with a margin | |
Although I’m not sure that I’ve taken the measurement correctly, I couldn’t come up with a better way than turning the digital watt meter into the gap between the battery and the UPS. I had doubts about the correctness of measurements due to the fact that despite the constantly on load, the UPS consumed current for periods (3-5 seconds the consumption increases to the nominal value and drops to zero, 1-2 seconds there is no consumption), maybe this is due to the fact that a pair of capacitive capacitors are installed at the battery input, which smooth the load on the battery. Charging is performed in approximately the same way (a current is applied for a while, then a couple of seconds break). After a full charge, the UPS continues to periodically supply current to the battery in the region of 1A.
Despite the fact that one uninterruptible mercilessly raped the batteries every day almost completely discharging them, and then recharging them, and the second worked in the normal mode and discharged the battery only when there was a power outage, after a year they still work and hold the load. Specialized batteries in the UPS that stood from the factory, that were purchased during operation did not live with me even this time, they simply dried up and stopped holding the declared capacity. In general, I could not answer for myself the question of why car batteries are not suitable for use in UPSs, but after a year I will try to repeat the measurements and compare the results.
PS Before measuring the capacity of battery No. 2, I managed to short it with pliers. Now they will serve as a good reminder that it is better not to do so. By the way, all pictures are clickable.
