Sprinter Battery: Upgraded 12V Electrical System
Article contributed by J.M.Fife
12V Battery: Project Background
4 months ago: “I think I will add a solar panel to my camper.” Somehow that turned into a pretty
extensive rework of the 12V electrical system. Oh well. It was a fun project and I learned a few things.
I doubt anyone will want to duplicate this mod, but I am documenting it for three reasons:
1. For the next owner in case they need to service it.
2. For curious people who might want to use a few pieces of this system.
3. Just because that’s what I am trained to do as an engineer.
Here is the summary…
Photovoltaic Generator, Charge Controller, and Battery Over-Discharge Protection
1. 2x100W flexible PV panels HQST-100DB (Amazon.com or Ebay.com $180 ea). The boomerang
TV antenna was removed to provide space. 10A fuse installed at the panels with MC4 holder.
Each module is 17.7V open circuit and has a built-in bypass diode. Wired the two panels in
series. Screwed down with generous caulk. I routed the wires same as the (now gone) TV
antenna. Once into the camper, I routed them down, to the back of the camper, and through to
the area where the original battery charger was located.
2. Victron MPPT 75/15 Charge Controller and VE Direct USB cable from PKYS.com $138. Great
a. Max. power: 200W.
b. Topology: maximum power point tracking. Peak efficiency 98%.
c. Adjustable behavior (such as charge voltages and low voltage cutoff points) via USB
interface to PC. I set the absorption voltage to 14.2V because of the lithium battery (see
d. Bluetooth connection to an “app” is also available.
e. Load voltage disconnect. I use this to power the EV-200 contactor to power down the
entire camper load if the battery voltage drops below 11.1V. This prevents overdischarge
of the battery.
3. SPDT PV Input Switchover Relay (American Zettler, Inc. AZ2280-1C-120AF Relay; E-Mech; Power;
SPDT; Cur-Rtg 30A; Ctrl-V 120AC; Vol-Rtg 277/28AC/DC; UL, CUR from Amazon.com $12). This
relay actuates when shore power is connected to switch from PV to DC supply. This way, the
Victron manages the battery charging when the camper is plugged into shore power, or when
the genset is running. I know the DC power supply is not a true PV source and has a very
different I-V curve, but I used to design PV inverters and know that most of them work just fine
with a DC power supply input. And yes, I can confirm the Victron works fine with a DC power
supply input as well.
4. Shore Power to DC Switching Power Supply. 120/240V to 24V, 15A, 350W from Walmart.com
$20. This DC power supply is powered by AC and generates 24V which goes into the PV Input
Switchover Relay. It also enables battery charging at the full rating of the Victron (200W),
whereas the original camper battery charger was only rated to 10A (130W at 13V).
5. Replace manual camper main disconnect with an electromechanical contactor and pushbutton
switch (EV200AAANA and GPB553B2L01BG from Digikey $130). Coil powered by the Victron
load port. So when the battery voltage drops below 11.1V, the Victron shuts down the load
port, and EV200 opens. This protects the batteries from over-discharge which is damaging to
6. DC Combo Meter Battery Monitor +/- 50A Voltage 0-20V from eBay.com $26. This is a
combined voltmeter and bipolar ammeter. It shows the total current from the battery (positive
means discharging and negative means charging).
7. Schottky Diode (DSS2X121-0045B from Digikey $20). Rated 45V, 2x120A. Installed between the
camper battery and the van battery. Keeps the van battery charged with the same 4-stage
charging regiment as the camper, but does not allow the van battery to discharge into the
camper load. There is a small voltage drop across the diode so the van battery doesn’t quite
reach the full 14.2V during equalization, but this is still way better than having to plug into shore
power to keep the van battery charged. Also we can now we can run the van accessories such as
stereo and hydronic heater without concern because the van battery gets recharged
continuously through the diode.
8. Various wire, hardware, fuses, connectors, terminal block, sealant, etc… $100 or so.
Hybrid Lithium-Ion + Lead-Acid Battery
Some background here… This idea came about because it seemed like my battery capacity was starting to fade. And I like to experiment with new ways of doing things. And I got obsessed… Why did I think my capacity was fading? On a mountain biking trip with some buddies, the heater started acting erratically because the voltage was dropping below the cutoff voltage. And I though it had the original gel cells. Well actually it has flooded lead-acid batteries that don’t really fit properly. But I tested their capacity and they were OK. I think the real issue was a loose ground connection. But I had already bought some parts for my new battery so I went ahead with the project. The upgraded battery configuration is a hybrid li-ion + lead-acid system. Why? Li-ion batteries have high cycles lives and can typically to cycles to 90% depth-of-dischage thousands of times before their capacity drops significantly. Lead acid batteries prefer to only go down to about 50% depth-of-discharge and can only do that a thousand times or so. And they can only go to 90% depth of discharge a few hundred times. And the roundtrip efficiency of lithium batteries is > 92% compared to ~70% for lead acid charge-discharge times of 10 hours. These are rough numbers. They depend a lot on temperature, charge profile, rate of charge and discharge, etc. But it should give you the general idea. The cycle profile for our campers is typically mostly shallow cycles and a few deep ones. So why not let a small li-ion battery handle the shallow cycles very efficiently, and let the lead-acid handle anything beyond that? Can we just directly connect the in parallel? Yes! But they are different chemistries so it requires correct voltage matching and safety protections. With 4s LiFePO4 + lead-acid, the system voltage is about the same as a lead acid battery, but the state of charge vs. voltage curve is much flatter than for lead-acid. This actually results in an excellent match for the purposes described above. See the data plotted below which shows each battery’s state-of-charge vs. voltage. In this test, we have a 100Ah lead-acid battery and 30Ah LiFePO4. You can see from the figure that as discharge occurs (going from the right to left on the graph), the li-ion capacity (yellow) is consumed first. Then it remains at 10-20% state-of-charge range while the lead-acid battery (blue) discharges. Here are the parts I used… 1. LiFePO4 battery configured 4 cells in parallel. I took an old Headway 10Ah 36V battery and reconfigured it to 3-parallel and 4-series (4s3p) ($350). 2. A battery management system that monitors each cell voltage and provides alarms and opens the safety relay if necessary. I used a Housepower BMS from CleanPowerAuto for $145. For an audible alarm, I added a mini buzzer with a resistor in series. Great product. 3. A safety relay (CB1AH-M-12V from Digikey), fuse holder, and 30A fuse. (I started with a cheap 30A breaker I got on Amazon.com but decided to test it to make sure it worked, and it didn’t open so I swapped it for a fuse – more reliable. The pics below show the original configuration with the breaker). 4. A high amperage quick disconnect (Driver Battery Quick Connect Plug Kit from Amazon.com) 120A, $16. 5. Foam rubber mounted under the battery for vibration isolation ($7 at the local hobby shop). 6. Various hardware, fuses, ring terminals, connectors, etc. (~$20) My original plan was to go with the 30Ah Li-Ion battery plus only one 100Ah battery. I was going to mount the L-Ion in the battery compartment alongside the remaining lead-acid. but once I realized my existing lead-acid batteries were both OK, I just left them both in and moved the Li-Ion to a position under the stove in the lower cabinet. So now I have 30Ah Li-Ion + 200Ah lead acid.
Sprinter Battery Upgrade Results
I love this modified system. We can park the camper at an airport and come back a week later to cold
beer. We can keep ice in the fridge indefinitely while dry camping.
I also strongly believe this setup will prolong the life of my batteries mainly because the lead-acid
batteries will see significantly less throughput than before. When the existing two 100Ah flooded lead-
acid batteries die of old age, I plan to replace them with a single 100Ah valve regulated 100Ah lead-acid
(VRLA) AGM. At that time, I will also move the li-ion battery to freed up space in the main battery
Sprinter Battery Upgrade Photos
30Ah li-ion (LiFePO4) battery with protection and balancing circuitry.
One of the original panels had a defect and burned through. Had to return it for a replacement.
2x100W flexible PV modules installed.
Installation of the li-ion battery.
Lead-acid battery compartment. Had to run 6AWG cable between the 2 lead-acid batteries to the li-ion
battery. It’s a pretty easy fish-tape from the electrical panel to the bottom cabinet where the li-ion
battery is now installed.
Added a shelf in the cabinet above the li-ion battery. Note the Battery Tender visible at the back of the
cabinet is disconnected now. I just haven’t removed it yet.
Revised electrical panel. DC power supply (left), MPPT charge controller (middle), shunt (top), Schottky diode (top right), and SPDT switchover relay (bottom right).
Readout of voltage and current. Pushbutton control of master relay. Instrumentation handles mainly to
prevent accidental actuation of the buttons like the OE sheet metal shield used to do. I just like this
better because it doesn’t take up as much of my cabinet space and looks like manned spacecraft
hardware from 1970.