Impact of a 12V tap off a 48V battery string

DustyDusty Solar Expert Posts: 271 ✭✭✭
I have two banks of 4, 12V AGM batteries powering my XW6048 for a total of 48V, 110Ah.

If I tapped off of one of the 12V batteries to power a 12DC lighting circuit, would there be any detrimental impact to the string in general? Since the batteries are in series, but I attached the lighting circuit to only one battery, I'd have 12V output to that circuit, but in that case, wouldn't all 4 batteries in the string contribute as though they were in parallel?  (actually, both 4-battery banks would contribute) For that 12V circuit, would I now have 440 Ah (8, 55Ah 12V batteries)?
XW6048, 3.4KW PV, Grid-Tied, always tweaking.


  • BB.BB. Super Moderators, Administrators Posts: 28,142 admin
    People really want to tap their higher voltage battery banks for 12 volts to run some smaller stuff...

    In general, this is really bad idea. Basically, you discharge the one battery, and have to Equalize (over charge) the other series connected battery to recharge the one 12 volt battery that has been tapped. With Lead Acid batteries, you can EQ/overcharge a little bit (flooded cell lead acid batteries are more forgiving--But you still really only want to pull very little current from such a setup).

    Other batteries (AGM, GEL, Li Ion) batteries are very unforgiving if you try to "EQ" them (AGM/GEL will overheat/vent; Li Ion cells will overvoltage and fail quickly, many Li Ion chemistries will explosively vent and/or catch fire).

    There are ways of getting 12 volts:
    • Use a DC to DC converter... A simple/cheap module will take higher DC voltage and down convert to 12 VDC. A very good way to do it, especially if you only need a few Amps of 12 volts.
    • For larger current needs, there are battery balancers--Typcially used on 24 VDC vehicles (bus, RV conversions) that connect between the Upper and Lower 12 volt batteries can "move current" between the two to keep them balanced. Works well, not cheap. Good for larger power needs.
    • An interesting option is to buy a smaller MPPT solar charge controller... Connect the Vpanel to the 24/48 volt bank, and use it to keep a 12 volt battery charged. HAM people have been happy with this approach (separate 12 volt battery for emergency backup, support high surge currents for higher power transmitters).
    • And the good old standby... If you already have a 48 volt system and a good size AC inverter--Just use an AC to DC power supply. Get a good quality one (efficient), and use it for your 12 VDC loads. Maybe something like 15% "extra losses" over DC to DC (vs DC to AC to DC)--One thing people tend to forget--A 12 VDC lead acid battery system typically runs between 10.5 and 16 volts (heavy loads/discharged to Charging/EQ charging). Not a lot of native DC devices like that wide of voltage range. Using your AC inverter as the "buffer" (or a DC to DC down converter), gives you a much more stable 12 VDC voltage.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DustyDusty Solar Expert Posts: 271 ✭✭✭
    Thank you Bill!  That's why I ask here before doing something ignorant. 

    I was thinking about adding a few low wattage 12V lights as emergency lighting.  I usually keep my vital loads (refrigerator and chest freezers) running off the 6048 at all times, and I'll kick in the secondary loads when PV is adequate to support them all.  But if I lose power in the middle of the night, it would be nice to have emergency 12V lighting that wouldn't pull much from my batteries. 

    I was trying to design a system that didn't incur any losses due to DC-to-DC or AC-to-DC conversion, which is why I considered tying directly to the battery bank.  But I did NOT consider the impact of equalization to my AGMs.  Thank you for pointing that out so clearly! 

    Currently, I have a few other 12V AGMs scattered around the house with small inverters attached to them for emergency lighting, but it just seemed an inefficient way to do it, since they all have independent AGM "Battery Minder" chargers attached to them all (I'm trying to decrease phantom loads); as well as the losses incurred in the charge and DC-to-AC conversion processes.

    I appreciate your advice!

    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • littleharbor2littleharbor2 Solar Expert Posts: 1,098 ✭✭✭✭
    How about a 12 volt panel, charge controller and battery in a separate 12 volt system?  Our place in Baja has 12 volt VHF radios, water pumps and stereo. Seems to work very well for us.  none of these items are a heavy load so a minimal system is all that is needed.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators, Administrators Posts: 28,142 admin
    Connect a Kill-a-Watt meter to some of the newer battery chargers and see what the phantom load really is...
    I have been very surpised that many of my newer electronics don't even crack the 1 Watt level when turned off/on standby... I have an older (nice for me) stereo that would burn 20+ Watts when "off" (just running the digital clock).

    For emergency lighting--LEDs + some sort of battery/rechargeable battery are hard to beat these days... Get a 3,000 mAH Li-Ion USB battery based backup to run something like this (0.1 to 0.2 amps @ 5 volts):

    And an 18650 Li Ion battery charger (many have USB power bank feature):

    Or just go with an 18650 single cell flashlight... I like those with (at least) several brightness levels... On low, they will run more than 100 hours (typical around 1 lumen or so--Just enough to walk around and read some text). And they are very bright on high (maybe an hour at 1,000 lumen output).

    A handful of "phantom power" Watts is not really going to been seen on a larger off grid solar power system.

    If you really want to get into the lighting/flashlight realm--These are a couple of good places to start:

    Or get a box of AA/AAA batteries and some LED flashlights (store the alkaline batteries outside of the flashlight--I have had too may leaking these days and ruin my nice LED lights).

    The low drain NiMH "Eneloop" batteries are very nice these days (use the multi-chemistry AAA thru 18650 chargers). I have had very good luck with them in low drain devices (remote controls, flashlights). They don't leak (at least so far).

    I would stay away from the "high capacity versions"--They tend to have higher self discharge and are not great for storage/emergency use.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DustyDusty Solar Expert Posts: 271 ✭✭✭
    edited July 28 #6
    Thanks Bill,

    I rigged up a WEMO Insight to my normal a/c input to the BatteryMinder charger for a single AGM battery and inverter setup. This AGM battery is tied to a 12V SunForce 2500w inverter, but for low wattage lighting, I'm using a small, 300w inverter plugged into the 12V pass-through cigarette lighter connection on the 2500w inverter to in turn power a 7W LED lamp.  I'm doing that because the SunForce pure sine wave inverter draws 1.9A with no load on it--more than the LED lamp it is powering.  The 300 watt inverter, turned on with no load only draws 0.2A.. With the lamp lit, the draw is 0.9A.

    The Wemo Insight tracks power just like a kill-a-watt meter, but it also allows me to program the Insight as a timer as well as logging all power use.  This battery system also has a Xantrex Lite battery monitor attached.

    I have the Wemo switch set up to only run 30 minutes a day to keep the 55Ah Optima Yellow-Top (D34) battery topped off.  The Wemo reported that when the switch was on and the BatteryMinder was charging the battery, it used an average of 96w for the 30 minutes it was running. (bulk charging at the time. The battery capacity was at 99.5 when the Wemo switch turned on).  That seems very high, since when I turned it on manually and watched it, it varied onlty between 9-11 watts. I'm not sure how they compute the average watts, but I'm more willing to believe the 9-11w reading.

    With the 300 watt inverter turned off, the Xantrex LinkLite battery monitor shows 0.0A draw.  When the 300 watt inverter is on and the lamp is lit, the draw is 0.9A.

    I've got 3 such AGM battery backup power systems throughout the house with battery minders attached running 24/7, as well as two more keeping my vehicle batteries topped off, and two more keeping my riding mower batteries topped off too.  So when I was searching for phantom loads, I knew that having all these battery chargers running 24/7 was contributing to my daily usage more significantly than I was giving them "credit" for.  But the alternative is dead batteries, and at $275 for a replacement AGM battery (I have AGMs in my car and pickup too), a few cents a day to keep them topped off is a small investment.  But I'm trying to see just how many hours a day my chargers really need to run to keep everything topped off without having to run them all 24/7. 

    I'm also using the 18650 Li-ion and Eneloop low drain AA and AAA batteries.  I use them a lot when camping. I've got several of the 18650 chargers that use a 12V input that I can connect to my 27W backpack folding solar panel.

    I was just trying to consolidate emergency lighting in the house instead of having all of these different independent flashlight battery and 12V battery sources cluttering up the house. 

    Thanks again!
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • DustyDusty Solar Expert Posts: 271 ✭✭✭
    edited July 28 #7
    How about a 12 volt panel, charge controller and battery in a separate 12 volt system?  Our place in Baja has 12 volt VHF radios, water pumps and stereo. Seems to work very well for us.  none of these items are a heavy load so a minimal system is all that is needed.

    I would love to do something like this, but I'm not sure how practical it would be to run separate power cable throughout the house for lighting.  I could certainly do something like this in my garage.  In fact, I've thought about something like this in my detached garage, since it gets no PV power since it's not on my 6048's sub-panel.  I'd like to keep all my cordless tool batteries topped off using something like what you are suggesting.  I'm thinking that just a couple solar panels and an inexpensive mppt controller would be enough to do the job. Thanks!
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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