Installing a KISAE DMT-1250 DC to DC charger

[CriftinsCampers]

I have been considering buying a DMT-1250 and a quick Google brought up this thread so I signed up to the forum just to thank the OP for the level of detail

A couple of observations maybe the European versions are different (I am in the UK) but please double check the solar max Vcc mentioned in your opening post?

I don't think it is as high as 45V which is a real pity because I tend to use 300W panels so 45v would be ideal!

My sprinter has a permanent parasitic load of 115mA so I would definitely need to use a relay to cut it off pronto, I need to preserve as much power in that one as poss!

[Scalf77]

The manual that I copied that info from https://www.donrowe.com/v/vspfiles/p...ners_guide.pdf page 12.

Also referenced 10 - 45 in spec https://www.donrowe.com/v/vspfiles/p...spec_sheet.pdf

I have only run 18.5 volt panels in parallel on my installation., so I can't validate if they actually do support 45 volts

-greg

[CriftinsCampers]

Ok thanks that’s very interesting, there’s a discrepancy in the uk they are being sold with a lower max Vcc your doc says 45v.

The online manual at kisae says 50v

https://www.kisaepower.com/wp-conten...evC_190516.pdf

The data sheet 45v but then says 35v nominal

https://www.kisaepower.com/wp-conten..._datasheet.pdf

And the product sales page says 32v then a bit further down a contradictory 35v that’s the bit most potential buyers will be reading

https://www.kisaepower.com/products/...1250/#tab-id-2

I will drop them a line, if it can take a Vcc up to 40v it will be ideal for the client install I am planning. But they may for some reason have derated the capacity on the latest models but more likely they simply have some version control differences in their tech pubs dept.

Again thanks for highlighting this possible extra capability

[JayR]

Thanks for a very detailed and informative post. Learned a great deal from your installation.

I am considering installing the DMT 1250 charger in my RV. Currently it has 2 lithium batteries*+ 1 AGM battery as its battery bank. It's recharged from a second alternator which I am planning on removing*cause*its placement been nothing but added headache and a source of tensions whenever I visit the dealership for service.

My question has to do with Battery Bank Size on page 7 of the manual. It says the battery charging current rating is based on the battery size and should meet the minimum Ah rating as shown. Is the battery size a value that needs to be inputed, and what happens if one of the lithium batteries BMS shuts off the battery?* If a smaller size battery bank is available would the current rating needs to readjusted to a lower value?

Really appreciate*any info you can provide. TIA

[rob_gendreau]

Originally Posted by JayR

Thanks for a very detailed and informative post. Learned a great deal from your installation.

I am considering installing the DMT 1250 charger in my RV. Currently it has 2 lithium batteries*+ 1 AGM battery as its battery bank. It's recharged from a second alternator which I am planning on removing*cause*its placement been nothing but added headache and a source of tensions whenever I visit the dealership for service.

I tried reaching out to Kisae customer service to help answer a question but received no response. My question has to do with Battery Bank Size on page 7 of the manual. It says the battery charging current rating is based on the battery size and should meet the minimum Ah rating as shown. Is the battery size a value that needs to be inputed, and what happens if one of the lithium batteries BMS shuts off the battery?* If a smaller size battery bank is available would the current rating needs to readjusted to a lower value?

Really appreciate*any info you can provide. TIA

I don't recall that the 1250 required a setting for the battery capacity, but it certainly does for type. And if your battery bank is as you say, both lithium and AGM, I don't think this would work very well since the charging parameters are so different. Even if the banks are separated by a switch you'd need to change the settings each time you changed to charging a different bank.

[Kibo]

I don't think you should be mixing battery technologies--not just from the perspective of charging voltages, as Rob has mentioned. The discharge voltage curve of a lithium is much different than AGM. LiFePO4 (I'm assuming) will discharge at a higher voltage and remain relatively constant over most of the SoC curve, whereas AGM will start lower and drop more quickly.

In fact, if you have your lithiums connected in parallel with your AGM, I think you'll find that the lithiums are discharging to the AGM battery (or said another way, charging the AGM) when there's no load.

As for settings, the Kisae is configured based on maximum charging amperage and shutoff (charging termination) current. These are selected based on battery bank size, but there is no setting for the bank capacity per se. In the case of LiFePO4 you can max out the charging rate of either Kisae model (30A or 50A) with a moderate sized battery pack. Charging rate is calculated based on the capacity: for example, a 100Ah battery charged with 50A is 50/100 = 0.5C.

You didn't state the capacity of your lithium batteries, but 0.5C is considered a 'safe' charging rate for LiFePO4 in general, so as long as they're each at least 100Ah you shouldn't have a problem if the BMS were to disconnect one from the charging bus. If your batteries are smaller, there's nothing wrong with choosing a lower max charging current--it just means it will take longer to charge, so you're not taking full advantage of lithium's fast charging capacity.

Note that having the AGM in parallel will greatly confuse things, so all of the above is based on a Li-only setup.

Shutoff/termination current will be dependent on your setup, meaning you can set it based on quiescent current draw, but Kisae's general recommendation is 1/10 of your max charging current. You can always err on the high side to be safe, it just means you might get 95% of full SoC instead of a full 100%--which is actually better for battery cycle life anyhow!

Last but not least, you'll need to set the charging voltage and (if used) float voltage. These settings differ for Li vs AGM, so again you really shouldn't be mixing technologies. It is generally recommended not to float Li at all, although I am experimenting with using a float voltage on my setup since the charging algorithm is externally controlled by my BMS.

[JayR]

Thank you both for the feedback and sorry for any confusion I am total newbie to the world of DC power. I completely misread the manual regarding the battery size. Thanks for clarifying it's more of a guide for choosing the appropriate current rather than setting a value.

Here's more background info about my setup.

* RV is a Hymer built (with Ecotrek system)
* 2 200Ah LiFePO4 lithium (Batteries by KS2 with full charge ~13.7V)
* 1 105Ah AGM
* 2nd alternator is 280Ah Nations Alt. and volt regulator is Balmar MC-614
* batteries wired in parallel
* 200W of Solar
* lithiums have 1C value and was going to stay <= .5C to prolong battery life

You are correct Kibo, currently the lithiums could discharge to the AGM though with solar and alternator as charging sources the AGM is always topped off. Also I never let the lithiums voltage drop below 13.0V

Lithium batteries can be turned off when full and not needed and the AGM will continue to provide DC power. I mostly use the AGM for running the fridge while driving in town with frequent stops to spare the lithiums reentry into bulk stage. Though AGM's main purpose is to absorb any spike in current if/when lithium shuts off while alternator is charging.

Sounds like keeping the AGM in the mix is a no go. Which begs the question what's the impact if the BMS decides to shut off the lithium in the middle of a charge cycle, would an alternator surge protector be required?

My initially thinking was to configure the DC charger for lithium type battery. I understand that lithium don't like float charging, so I was planning to have the "L" setting somewhere around 90-95% of a full lithium charge (13.5V) and the charger will terminate the charging process. Thinking this would still be safe for lithium & AGM.

Truly appreciate your insights and feedback, tc.

[Kibo]

Originally Posted by JayR

Sounds like keeping the AGM in the mix is a no go. Which begs the question what's the impact if the BMS decides to shut off the lithium in the middle of a charge cycle, would an alternator surge protector be required?

I think this is a valid concern. My approach to this is outlined in this earlier post.

Basically, I use the "charger enable" output from my BMS to disconnect the battery from the charging bus in case of an alarm/error condition by shutting off the Kisae via a relay wired in place of the manual on/off switch. This relay is wired in series through the alternator field terminals of my manual disconnect switch for the same reason, to shut down the Kisae before physically disconnecting the battery from the bus.

Assuming you remove the AGM altogether and wire the BMS output from each Li battery to relays in series in place of the Kisae manual on/off switch, you would have essentially the same functionality--but with the caveat that if either BMS were to signal an error condition, all charging (to both batteries) would be shut off.

In my setup this is intentional--the disconnect method is meant only as a safety interlock for error/alarm conditions, not to control the charging process. It sounds like you want the charging to be controlled by the Kisae's algorithm, but I figured I should point this out in case my assumption is wrong.

[JayR]

I heard back from Kisae and they couldn't be more helpful and forthcoming with info. The voltage on the CH1 output port (and not the one from the starter battery) is used to supply power to the control electronics inside the DMT1250. If there no house battery voltage going to the CH1 output port (as a result of BMS shutdowns) the DMT1250 just turns off automatically and remains so until that voltage is restored again.

As for automatically turning the charger on/off as you pointed there are different ways to implement this. The rep also suggested using the BTS jack connector by forcing a fake Battery Over-temperature Alarm (error code E07). Always good to have options.

[Kibo]

That's some good info, thanks for sharing! Personally I'd still be concerned about the ramifications of suddenly removing the battery (load), but the BTS jack sounds like a less invasive way to implement this.

It occurred to me that this link could have been helpful to understand my previous posts regarding the use of the alternator field disconnect (AFD) terminal on my Blue Sea battery switch:

Blue Sea AFD

Without knowing the internal design of the Kisae, this seemed like the safest option at the time. Using the BTS jack never occurred to me--I feel kind of stupid now!