How to measure the AC ripple of the DC from a Bluetti EB55?

I’m trying to use an EB55 to power a Truma Combi D6 (diesel) heater (for a campervan) and I’m getting this error when I try to start it up: “12V power supply faulty.” Using a simple voltmeter, I’ve confirmed that 12V is being delivered by the EB55 DC supply.

In the error codes and installation manual it says to ensure the “alternating current ripple is < 1.2 Vpp”

Is there a way I can measure the ac ripple on my EB55 without an oscilloscope?

Below I’m including the exact quotes from the Truma error codes / installation manual.



Ensure 12 V power supply under load at appliance input / Check for AC voltage components (disconnect power supply unit and solar system from on-board power supply)

When power packs or power supply units are being used, note that the output voltage is between 11 V and 15 V and the alternating current ripple is < 1.2 Vpp. We recommend the automatic chargers from Truma for the various applications. Please contact your dealer. Only use chargers with a 12 V battery as a buffer.

It sounds to me like your Truma heater is drawing more power upon start up than the EB55 can supply.

This reply makes a lot of sense, and it’s part of the reason I’m writing to a Bluetti forum. The Technical Data portion of the Truma Combi D6 installation manual has the following:

Current input at 12 V
Heater + hot water system 1.8 – 7 A (in regular operation, between “Off” and “lowest operating level” less than 1.8 A). Heating hot water system without operational heater, max. 1.8 A

This diesel heater can supply either hot air or hot water or both. My understanding is that the initial startup of the heater requires a lot of electrical power but then once ignited very little electricity is needed. For this reason relatively large wires (4mm2 to 6mm2) are specified to connect the heater to the 12V power supply. I’m using 6mm2

It seems to me that the Bluetti EB55 spec would allow it to be used for this heater. I’m not sure what I’m missing so I’m looking for things to test to figure out where the problem is. My next step is to contact a Truma representative and because of the error message I’m guessing he’ll say it’s a problem with the power supply.

It sounds to me like your Truma heater is drawing more power upon start up than the EB55 can supply.