Hi, I’m still learning, so this may seem to be a dumb question to some of you, but I think it’s valid. ![Eek! :eek: :eek:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I am building a small “general purpose” 12-volt Uninterruptable Power Supply (UPS) that allows for any low current 12 volt device to be hooked up to it (up to 1A, depending on the battery and input power source/regulator).
I have an LM317 setup that keeps an SLA battery on float charge, and also powers the 12-volt device at the “Load Output” at the same time. Should the mains power fail, the battery keeps the load powered as a backup.
When the external load is switched off/not connected, voltage from the LM317 to the battery sits comfortably around 13.5V, which is normal for float charging. But connecting a load causes the voltage to the battery to drop down a fair bit (for example, from 13.5 to 13.0V). I know there’s got to be a voltage drop somewhere whenever a load is connected (and how much current it’s consuming), but, doesn’t that mean the battery will suffer? I was told a while back from somebody that the voltage drop is normal, and that the battery will be fine, as long as it is above 12.6V with the load connected (or somewhere along those lines), but I really would like someone to confirm that.
^^ When mains power fails, I have an N.C. relay contact that shunts/bypasses the 5W resistor so that there is minimal current loss from the battery to the load. Load will automatically be disconnected if voltage falls to 11.8V.
I would appreciate any assistance / feedback on my current design (see attachment).
JASON.
I am building a small “general purpose” 12-volt Uninterruptable Power Supply (UPS) that allows for any low current 12 volt device to be hooked up to it (up to 1A, depending on the battery and input power source/regulator).
I have an LM317 setup that keeps an SLA battery on float charge, and also powers the 12-volt device at the “Load Output” at the same time. Should the mains power fail, the battery keeps the load powered as a backup.
When the external load is switched off/not connected, voltage from the LM317 to the battery sits comfortably around 13.5V, which is normal for float charging. But connecting a load causes the voltage to the battery to drop down a fair bit (for example, from 13.5 to 13.0V). I know there’s got to be a voltage drop somewhere whenever a load is connected (and how much current it’s consuming), but, doesn’t that mean the battery will suffer? I was told a while back from somebody that the voltage drop is normal, and that the battery will be fine, as long as it is above 12.6V with the load connected (or somewhere along those lines), but I really would like someone to confirm that.
- Input: 18V DC (Regulated Switchmode type).
- Regulator: LM317 (1.5A); voltage output set to 14.2V (before series diode; 14.2 – 0.7 = 13.5).
- Current limiting resistor to the battery: 4.7R, 5W (up to 0.36A @ 11.8V)^^.
- Battery: 12V SLA, 2.2Ah (0.66A max charging current).
- Load (external device): Up to 1A (absolute maximum).
^^ When mains power fails, I have an N.C. relay contact that shunts/bypasses the 5W resistor so that there is minimal current loss from the battery to the load. Load will automatically be disconnected if voltage falls to 11.8V.
I would appreciate any assistance / feedback on my current design (see attachment).
JASON.
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