Test/review of Charger Hohm Tech School4

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HKJ
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Test/review of Charger Hohm Tech School4

Hohm Tech School4
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A four slot LiIon only charger from Hohm Tech that is USB powered.
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The charger was in a cardboard with specifications on it.
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The box included the charger, a USB cable and a instruction sheet.
The USB cable is very thick.
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The charger has a USB type-C input. It only uses 5V and the supplied cable is with a USB-A plug in the other end.
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There are no buttons, only a led for each slot. The leds is red while charging and green when the battery is full.
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There is two current settings on the charger and they are selected by the battery length. There is a nice marking showing at what point the current changes.
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The charger has specifications on the back.
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The connection are the common slider style that can handle from 32mm to 76mm, this means all batteries will fit.
supportedBatteryTypes supportedBatterySizes
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The current for 10440 is a bit on the high side. The charger is not rated for 32xxx batteries, but LiIon types may work in it.
Measurements charger


  • When not powered it will discharge a LiIon battery with about 10mA
  • When powered the charge will charge with less than 0.5mA when finished.
  • At 0V the battery is assumed to defect and the red indicator blinks.
  • Between 0.5V and 2.8V the charger will use reduced charge current
  • Above 2.8V the charger will use full current.
  • Charger will not restart if battery voltage drops.
  • Current changes around 62mm battery length as marked on the charger.
  • Slot #1 & #4 will charge with 1A or 2A when slot #2 & #3 is not used, but only 0.5A & 1A when they are used.
  • Slot #2 & #3 will charge with 0.5A and 1A
  • Power consumption when idle without batteries is 13.3mA from USB

Charging LiIon
HOHM%20School4%20%28SA18650-33%29%20%231
Slot #1 uses two ampere with long batteries, the charge current is a nice CC/CV curve and a termination current around 130mA
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With slot #2 & #3 the current is 1A and the termination current around the same.
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And slot #4 is also 2A
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HOHM%20School4%20%28PA18650-31%29%20%232
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Doing these other batteries in a 1A slot worked nicely
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A 2A slot also works, but it harder on the batteries
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The small batteries is charged in a 0.5A slot.
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Here I use a 2A slot for charging a high current battery.
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Using a 0.5ohm resistor in series with the power supply to simulate a long cable or weak supply, the charger handles it nicely.
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The charger will charge four batteries fairly fast, but it do not play nice with the USB charge. In this case I am drawing nearly 4A from my supply, as can be seen above the charger will reduce current if the USB charger has trouble keeping the voltage up (The specifications says it can use 4A from USB).
Temp7576
M1: 36.6°C, M2: 41.0°C, M3: 39.8°C, M4: 34.5°C, HS1: 47.9°C
Temp7577
HS1: 45.1°C
PowerOnLiIon
The charger needs about 10 seconds to reach full current. I wonder about the initial pulse, it is at the full slot current, but the second time the current goes up it will stop at the low current setting if a short battery is in the charger (Two seconds at double charge current is not really an issue for any battery).
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The charger will discharge a full LiIon battery with about 10mA when not powered.
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The charge profile.
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The charger will not work reliable with a unstable voltage.
Conclusion
The charger can handle a wide variety of LiIon battery sizes and currents with currents selected from 0.5A, 1A and 2A it is very universal.
But it requires a bit much from the USB charger, it is best to use a charger with two or more USB outputs is parallel, that can deliver 4A or more (Check my USB charger tests). This also makes it a fairly fast USB powered charger.
I will rate it as good.
Notes
The charger was supplied by a Hohm for review.
Here is an explanation on how I did the above charge curves: How do I test a charger
My USB charger tests

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/