Upgrading UPS battery

I have a small UPS (APC Back-UPS 350) that has a 12 volt 7aH battery. I use it to keep my router and cable modem up and running in case of a power outage. I recently got a couple of Group 24 wheelchair deep cycle batteries. Can I connect these in parallel to the 12 volt battery in the unit to greatly improve the up time of the UPS? Do I need to provide a seperate battery charger/tender for the Group 24s and somehow "isolate" them from the UPS? What other considerations are there? I don't want to burn up the UPS.

I do not think the excessive batteries will cause a burn to the UPS BUT you have to provide an EXTERNAL CHARGER for these batteries as the Internal charger of this UPS in capable of charging only single 7AH battery. Assuming that the additional batteries (group of 24 but 12 volts) are connected to supply 12 Volts and not 24 volts. Once the charger is separate this is Isolation from the UPS, as you can can Isolate the batteries to supply the current to the circuit.

But i have a doubt ?
is the same charger can use as charger but it will take time toooo ?

is the same charger can use as charger but it will take time toooo ?

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No the built in charger is capable just for supplying 1 ampere current at 14~15 volts DC. It will burn out if connected to this BANK OF BATTERIES. APC has many versions but BK250, BK300, BK400, BK500 and BK600 have almost same circuit. The built in transformer is just 1200 mA and with 1N4004 rectification LM317 is responsible for charging the battery.

So the charging circuit is just a voltage regulator?
Isn't there a current limiter that controls the charge current?

Alex

Adding batteries will not affect charger.Charger need not to be replaced it will output the rated current shown on its casing adding batteries will result in increase of charging time for the batteries .

Isolation for charging is not necessary but not sure weather isolation for UPS output is necessary

It will be better to use one Group 24 battery alone instead of paralleling the existing UPS battery. This is because, since they are both 12V, there is no way of controlling how the charging and discharging (load) currents will be divided between the two if they are simply connected in parallel. And since they are of very different capacities, there is no way of ensuring that each battery will deliver a load current or receive a charging current that is in proportion to its Ah rating.

As others have pointed out, the Group 24 battery will take much longer to charge up than the built-in battery - at least 10 times longer. Of course it will power the UPS 10 times longer too.

The thermal design may be match to to the small UPS battery capacity. Normal operational on UPS time typical 20 minutes. By increasing the battery capacity (Ah) the UPS on time can be extended to hours ; but will the UPS thermal design be OK? (heat build-up)

klystron has brought up a valid point and I considered it too. But since the OP said that he's using the UPS to power only a router and a modem, it's presumably a very light load for the UPS and the power dissipated shouldn't be much more than at idle.

Pjdd,

I understand your point about the 2 different aH ratings of the batteries. Since I have 2 identical Group 24 batteries, could I connect them in parallel and eliminate the small 7aH battery. Also, are you saying I can use the charger in the UPS to recharge them after a power outage? I know it will take a long time to recharge them but would that be a problem? Finally, since there seems to be some concern about heat dissapation with extended power outage, would it be possible to wire in a cooling fan that possbly could be thermostatically controlled?

holabr said:

Pjdd,

I understand your point about the 2 different aH ratings of the batteries. Since I have 2 identical Group 24 batteries, could I connect them in parallel and eliminate the small 7aH battery.

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I don't see any problem with eliminating the 7Ah battery. As to paralleling the two Group 24s, basic theory says that it's OK. But in practice, small differences in the two batteries' characteristics, especially as they age, could lead to unequal sharing of the charging and load currents.

Also, are you saying I can use the charger in the UPS to recharge them after a power outage? I know it will take a long time to recharge them but would that be a problem?

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There shouldn't be any problem. In fact, the UPS will keep the battery topped up as long as it's receiving mains power, just as it does with its internal battery.

Finally, since there seems to be some concern about heat dissapation with extended power outage, would it be possible to wire in a cooling fan that possbly could be thermostatically controlled?

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Certainly. I don't really anticipate the need with such a light load but it wouldn't hurt. You could use a 12V 80mm computer case fan. I did the same thing for a friend who runs an internet cafe, but he was running several 1kVA UPSes to power a dozen desktops with CRT monitors.

So the charging circuit is just a voltage regulator?
Isn't there a current limiter that controls the charge current?

Alex

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Yes it is just a regulator. Here is the input circuit attached.


The user says he needs only an ampere or so to keep the Router and Cable modem ON.

I have a small UPS (APC Back-UPS 350) that has a 12 volt 7aH battery. I use it to keep my router and cable modem up and running in case of a power outage.

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I do not think this will over heat the UPS, as I am using the same model with an 80AH battery to keep my system running for quite some time without any problem.
The internal charger is capable of supplying maximum 1 ampere current to charge the internal battery hence an external charger is a must for these externally connected batteries otherwise the over current will cause the internal charger to burn out.

I don't understand, lead acid batteries are supposed to need a constant current cycle in the first stage of the charge, how does a constant voltage method like the above provide a proper charge?
I'm not experienced at all with USP so I'm just asking these questions to help me understand the circuit operation.



I did a search for LM317 lead acid charger but the circuits I found included some kind of current limiting

Alex, your unbderstanding is correct and I'm afraid I have to disagree with Raza about charging a battery with a regulated constant-voltage supply. There are so-called constant voltage battery chargers but it means something different from a regulated supply with a low impedance output.

To understand how a constant voltage (CV) charger works, let's first examine the other type which is a constant-current (CC) charger. As we keep charging a battery, the voltage gradually rises and the voltage difference between the battery and the charging source voltage decreases. But a CC charger regulates the current so that the charging current remains the same. This is preferable but requires a more complex charging circuit, making the charger more expensive, especially in the early days of solid-state technology when we didn't have a wide range of power devices and control ICs to choose from.

By contrast, a CV charger provides a constant voltage somewhat higher than the fully charged battery voltage. The charging current is set by the equivalent resistance between the source and the battery. This equivalent resistance may be a combination of a separate resistor and the internal resistance of the transformer. As the battery voltage rises, the difference between the source and the battery voltage decreases, and if the circuit resistance remains the same, the charging current also decreases.

Not long ago, I posted an explanation of how a simple automobile battery charger works and included a diagram. I don't remember the name of the thread, but you could search for it if you like.

Alex, your unbderstanding is correct and I'm afraid I have to disagree with Raza about charging a battery with a regulated constant-voltage supply. There are so-called constant voltage battery chargers but it means something different from a regulated supply with a low impedance output.

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pjdd,
I have not designed this circuit. This is from APC (American Power Conversion) company. I am having atleast 5 of this models at my home and I am myself a service person and repairing these. The one I said I am using for my system is powered originally with 12 volts, 7.5 AH (Sealed GEL) battery (Model # of battery ASB-1275 F2 by American Standard battery Co.).
I have posted the mains input circuit of the same from the original diagram. If you are interested I can upload the PCB photo of the same.

I was referrig to the idea that it's a voltage regulator. You have not shown the regulator type number so there's no way of knowing if it's a simple voltage regulator or a current regulator. A conventional voltage regulator with a constant voltage output is not at all suitable for charging a battery. The only way it can be used is to use its internal current limiter to set the charging current. For example, a 78xx IC has a built-in overload protection that limits the output current to about 2.2A. But then it is no longer functioning as a voltage regulator.

I'm aware of APC and knew about their reputation as makers of good UPSes long before they started marketing their products in eastern countries.

But in post #4 Raza has mentioned that a LM317 is used for charging although the wiring of the schematic in post #12 doesn't resemble a LM317 scheme because the gnd in tied directly in the regulator.

Sorry alex,
They have many versions of the PCBs for same model and I posted from a wrong referred PCB. Here is the other one.

No. it doesn't look like an LM317. The LM317 can be configured as a current regulator and is suitable for use as a charging current controller then, but that schematic is not that of an LM317. Besides, an LM317 needs a minimum drop-out voltage of 2V at 1A as a voltage regulator, and an additional 1.25V as a current regulator, making a minimum input-output differential of 3.25V.

Pjdd,

No. it doesn't look like an LM317. The LM317 can be configured as a current regulator and is suitable for use as a charging current controller then, but that schematic is not that of an LM317

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Please look the diagram in post # 18. It is clearly marked as LM317.