Battery supply, how to?

[Finnogangela]

Hi

I have a pair of (fairly) low-amperage devices I´d like to power with SLA-batteries;

- a 12V/7,2 Ah powering an Alix 3d2 motherboard (not sure of amperage but it consumes 3-5Watts with a 12V powerbrick (unknown effiency))

- and a 6V/12Ah battery for a QNKTC Usb-Dac (thus currently running on 5V ( 4.75-5.25V ok, 6V maximum according to manufacturer) amperage 24.4mA measured by user

1. challenge: How/best way to lower the voltage from the 6V battery?

(I measure 7.9V across the terminals fully charged..)

I know this can be done with on or more resistors or diodes in series with the battery - or with a regulator.

Any recommendations/other solutions?

 

[H E Pennypacker]

If the battery actually measured 6v, the device likely has an internal regulator to dissipate the extra volt.

An average SS diode would drop 0.7 volts, so in theory you could string a few together.

However if you find that your SLA pack holds that voltage for a while, (>6V) then perhaps consider rebuilding the pack without one of the cells. Might be best to measure the battery voltage again with a similar load connected to it.

 

[Finnogangela]

If the battery actually measured 6v, the device likely has an internal regulator to dissipate the extra volt.

An average SS diode would drop 0.7 volts, so in theory you could string a few together.

However if you find that your SLA pack holds that voltage for a while, (>6V) then perhaps consider rebuilding the pack without one of the cells. Might be best to measure the battery voltage again with a similar load connected to it.

The Dac is currently running on Usb-power (=5V)
according to manufacturer 4.75-5.25V is optimal, 6V maximum
amperage is 24.4mA measured by a user

The SLA battery measures 7.9V across the terminals fully charged (no load but I don´t expect to see any significant voltagedrop drawing 24.4mA from it..)

The battery is sealed, thus not possible to remove one cell: (here´s the 12V equivalent):



So, might try with 3 diodes in series

- or perhaps 4x1.2V NiMH will do (can see from voltage charts that they stay above 1.2V most of their charged time...)

 

[KatieandDad]

At such low current why not build a simple shunt regulator with a resistor and a zener diode.

 

[Finnogangela]

At such low current why not build a simple shunt regulator with a resistor and a zener diode.

Probably a good idea 🙂

Reading up on their function (found a nifty calculator here:

Zener Diode Voltage Regulator - Electric Circuit

- just need to understand what´s going to happen when the battery voltage lowers...

 

[Mooly]

It all sounds to marginal to me. The 12 volt battery with a reg down to 5 volts is the "best" solution. Your 7.9 volts (that's high... you are cooking the battery at that) will drop down to nearer 6.55 once its stood for 24 hours off charge. Series diodes from the battery will give a supply that varies as the battery discharges.

Also... at such low currents its not good to have the battery standing partially charged for long periods of time. That will cause rapid deterioration.

 

[BZed]

I would suggest a LDO series regulator as a less wasteful option. Shunt regulators waste power and reduce battery life. Several vendors now have series regulators that work with about 1 volt required for regulation and all of them have 5VDC models.

 

[Finnogangela]

Mooly; yes the 7.9 seems high to me too but that´s what it holds also here a couple of days after being charged (just measured). The 12V battery, charged with intelligent trickle charger, I measure to 16.9V...

Mooly, BZed: Actually I have an adjustable LT1764 reg. :http://www.audiowind.com/pdf/A-270.pdf (Vout=VIn-0.5V)

Will of course be simplest to try that with either of the batteries.

 

[BZed]

Those battery voltages are very high. I think I would make sure the volt meter is calibrated or try a different meter. That one is not correct. That battery should trickle charge @ 13.6 - 13.8 volts and fast charge @ no more than 14.6 volts. Higher than that and the battery will out gas and be damaged.

BZ

 

[speedless]

Hi

Those battery voltages are very high.

Yes!
Did you have the charger connected while measuring ?
I would use lithium batteries,specially at this low current.
They are more liniar when discharged,much smaller,lighter...
Have a look at this:
RC hobby
LI-PO batterier SUNHIGH lipo Hyperion Kokam
Fartregulator / ESC til brstelse motorer
Im planning to try the 5v BEC for my new motor-ptm.

Jan

PS:The QNKTC DAC looks interesting

 

[Finnogangela]

BZed, Jan:

Yes, will try and borrow another meter before pursuing (charger not connected when measuring).

Jan: Yes but having the SLA lying around I thought I´d try.

For clean battery power I think I´ll have a go with 4x1.2 NiMH (whem I´m able to measure reliantly).

Then, maybe for comparison, the SLA with regulator.

Yes, the DAC gets a lot of praise, even on USB-power and I like it too 🙂 (so I think it´s worth the effort with cleaner power),

 

[speedless]

Hi
Just a tought,wouldnt a regulator bring you back to the "unclean" USB power ?
4*NiCd will give you between 4.8V-5.something,under 6V.
No need for a regulator.

Jan

 

[BZed]

Not at all. The noise found on the 5 volts supplied by the USB is caused by the current demands of the computer. The 5 volts supplied by a regulator dropping the voltage from a battery supply with no other load would be very clean.