Author: overkillsolar

This is a Portable battery pack that we put together for demonstrations. We found this unused Bosch tool case that was just the right size for a Tesla Model S battery module with the BMS installed. It has a power switch on the outside which is connected to the BMS’s SW input, and the indicator light is connected to the 24v output (C- to B+). The output connector is a 120 amp Anderson connector mounted flush on the end, and we attached matching connectors to a suitable power inverter and charger.

This setup provides 3.6kwh of portable power that looks right at home on any job site.

Because 6 cell Lithium-Ion batteries have an end-of-cycle voltage of 18v, inverters designed for 24v systems may cut off early, reducing the available energy from the battery.

In this experiment, we connected 2 different 24v pure sine inverters to a used Tesla Model S battery module, and checked the power consumption of a variety of constant loads.

Note that the 2 inverters have different non-adjustable cutoff voltages.

The AIMS documentation specifies “Input under-voltage alarm 19.6 ± 1VDC”, and as tested it alarms at 19.5v, which is at 5% SOC on the test battery.

The WZRELB documentation specifies “Low Voltage Alarm 19.5-21.5v”, and it started beeping at 20.3v, which is at 18% SOC.

The cutoff voltage was tested using a variable DC power supply and a 23w light bulb loading the inverters.

Conclusions

This demonstrates that careful selection of your equipment will help get the most out of your
used tesla battery, without paying a premium price for programmable inverters.

The inverter with a lower cutoff allows you to utilize the full capacity of your battery.

On the other hand, you may prefer the inverter with a higher cutoff, which may extend the cycle life of your battery by not deeply discharging it.

A few surprises stand out in the test data.

First, the microwave oven rated “1150 Watts” could not be started using the 1,500 Watt inverter. When powered by the larger inverter, it pulled 1,888 Watts from the battery, nearly twice the power on the sticker! This is likely due to a large reactive component in the current flow used by the microwave. Because we measured the load using actual DC power flow from the battery, the extra energy must be dissipated by either the inverter or the oven itself. Further testing is needed to determine the most efficient way of powering a microwave oven on inverter power.

The 20″ floor fan only consumed 20% more power at high speed vs low speed, but it moved a lot more air. This fan uses a shaded pole induction motor, which is inefficient at low speed. Running one fan at high speed is much more efficient than running 2 fans at low speed.

Another interesting data point is the hair dryer. It consumed 86 Watts more when running on the AIMS inverter (which is actually 388 watts more than the rated output). This is because the AIMS inverter was supplying 117 Vac, versus 115 Vac from the WZRELB inverter. Since the hair dryer is a nearly 100% resistive load, it’s power consumption is directly proportional to the RMS AC voltage. Most of the other items tested showed little variation between the 2 inverters.

Test Data

Table 1: Average run time using the AIMS 1500w pure sign inverter

As tested, the AIMS 1500w pure sine inverter alarms at 19.5v, which is at 5% SOC of the Tesla battery.
The Tesla Battery will deliver 3450 watt-hours before this inverter’s cutoff voltage.
Wattage is measured from battery draw, including inverter efficiency losses.

Device	Actual Watts	Run time until inverter cutoff at 5% SOC
Inverter only, no load. (standby power)	16W	215.6 h
100w equivalent led lamp	37W	93.2 h
60w equivalent led lamp	25W	138.0 h
Small Air compressor (note 1)	570W	6.1 h
Desktop Computer (note 2)	250W	13.8 h
Hair Dryer, labeled “1875”	1,888W	1.8 h
Vacuum Cleaner, (note 5)	1,230W	2.8 h
20″ floor fan, high speed	160W	21.6 h
20″ floor fan, low speed	136W	25.4 h

Table 2: Average run time Using the WZRELB 2500W pure sign inverter

As tested, the WZRELB 2500W pure sine inverter alarms at 20.3v, which is at 18% SOC of the Tesla battery.
The Tesla Battery will deliver 2,987 watt-hours before this inverter’s cutoff voltage.
Wattage is measured from battery draw, including inverter efficiency losses.

Device	Actual Watts	Run Time until inverter cutoff at 18% SOC
Inverter only, no load. (standby power)	18W	165.9 h
Microwave oven rated 1150W (note 4)	2,080W	1.4 h
5,000 BTU window AC (note 3)	430W	6.9 h
Laptop charger rated 64w	81W	36.9 h
55″ TV and sound bar w/sub (note 6)	150W	19.9 h
Hair Dryer, labeled “1875W”	1,802W	1.7 h
Vacuum Cleaner, (note 5)	1,230W	2.4 h
20″ floor fan, high speed	167W	17.9 h
20″ floor fan, low speed	140W	21.3 h

Notes:

1. California Air Tools 1P1060S, rated 4.5 amps 2. Small form factor desktop PC, Windows 10, Intel i7, idle, with 3 monitors, plugged into an APC UPS 3. Cool-Living CLW-15C1A-JA09AC window air conditioner. 5000BTU/h, rated 4.0 amps 4. GE Microwave JES1657SM1SS, Rated cooking power 1150 watts 5. Shark NV70 31, Upright household vacuum with brush roll powered, rated 10 amps 6. Element 55″ Roku TV and old Vizio soundbar with bluetooth surround and subwoofer, playing Bob’s Burgers at party volume.

External product links on this page do not contain affiliate trackers- we will not make a profit if you buy either inverter.

LiFePO4 is the chemical formula for the cathode material in a Lithium-Iron-Phosphate battery.

Lithium iron phosphate exists naturally in the form of the mineral triphylite.

LiFePO4 is sometimes abbreviated as LFP.

LiFePO4 chemistry offers a considerably longer cycle life than other lithium-ion chemistries. Under most conditions it supports more than 3,000 cycles, and under optimal conditions it supports more than 10,000 cycles. NMC (Lithium-Ion) batteries support about 1,000 to 2,300 cycles, depending on conditions.

LiFePO4 cells experience a slower rate of capacity loss (a.k.a. greater calendar-life) than lithium-ion battery chemistries.

The major differences between LiFePO4 batteries and other lithium ion battery types is that LiFePO4 batteries contain no cobalt (removing ethical and economic questions about cobalt’s availability) and have a flat discharge curve.

Please check your cell polarity with a volt meter before connecting the BMS!

This type of cell has been shipped by other suppliers with the labels on backwards.

We have seen an excessive number of BMSs ruined recently because the customer’s cells had the labels on backwards

Most cheap prismatic cells on the market have black plastic on the POSITIVE terminals and beige plastic on the NEGATIVE terminals.

Note: This is not our product. Overkill Solar has never supplied this type of cell.

Yes, the BMS has a built-in balancing function.

HOWEVER no, it is not capable of doing an initial balance on new cells.

The balancer works by connecting a tiny bleed resistor to the cells with the highest voltage, and the excess energy in those cells turns into waste heat. This is a slow process. The intention is that the BMS can maintain the balance on the cells as they slowly drift over their lifetime.

A batch of new cells needs to be top-balanced before they can be expected to charge properly as a battery pack.

Why top balance?

Why? Because of the nature of the LiFePO4 voltage curve. At the top end of a charge cycle, the cell voltage spikes quickly, and charging must be stopped to prevent damage to the cells. If one cell is at a higher state of charge, (in terms of amp-hours or coulombs), even by a small amount, it will spike while the other cells are still in the “bulk” phase of their charge cycle. On the graph below, the red line is the highest cell, whic