Question about MPP Solar and Float Voltage - not necessarily MPP Solar specific

AREA351

New Member
Charging my battery pack for the first time and the MPP Solar seems to only get to the float voltage setting and then stops charging.

I have the Bulk charging set to 28V and the float set to 27.2V

I have the BMS set to 80% duty cycle (Overkill Solar) and the cells are showing 3.4V which is 80% capacity. Changing the duty cycle to 100% does not make any difference and the MPP Solar still only charges up the 27.2 and stops.

Am I missing something here? If the float voltage is set lower than the bulk, how can the pack ever be charged to 100%?
 

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snoobler

Solar Honey Badger
Moderator
There are multiple posts concerning this issue with some MPP Solar/Growatt units. If you're charging from AC power, this may indeed be normal as the AC-charger behavior is different from the solar charger behavior.

In my own personal testing of 40Ah CALB cells, I found that charging only to 3.4V/cell down to a .03C current (8.4A in your case) will achieve about 92-94% SoC.

Check the bottom of the page for this unit for a description:

 

AREA351

New Member
Thanks for that Snoobler.

I just got off the phone with Ian from Watts247 and he suggested to set the bulk and float charging volts to the same level. I will do this to fully charge the bank and do a capacity test and once complete, adjust both to where I would like to top off the battery to.

I'm not hooked up to Solar just yet.

You're saying that this only happens when using the built in utility charger correct?
 
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snoobler

Solar Honey Badger
Moderator
According to those comments at the end of the growatt, it appears to only apply to utility charging. Solar should follow the bulk, absorption, float profile.
 

Csimmer

New Member
I have a similar issue, MS5048. I have tried various combinations of float and bulk voltages and there is no great combination. Either the BMS gets triggered by high disconnect or the charging rate is way too low when the float kicks in way too early and because each charging day is different (sun all day, cloudy in the morning, cloudy in the afternoon) you can't find an optimal setting for all days.
I have decided to let the charger stay on bulk and control the charging with a bms. I set my bulk charge to a higher safe level and then adjust my bms high pack disconnect voltage to my target pack voltage - no float.
The problem with this is the high voltage spike that occurs when the charger is disconnected by the BMS, if I can reduce the spike it would give the best charging result.
 

Cruzian3

New Member
Thanks for that Snoobler.

I just got off the phone with Ian from Watts247 and he suggested to set the bulk and float charging volts to the same level. I will do this to fully charge the bank and do a capacity test and once complete, adjust both to where I would like to top off the battery to.

I'm not hooked up to Solar just yet.

You're saying that this only happens when using the built in utility charger correct?
I'm having the same issue. Have you tried setting both bulk and float to the same level? Any luck?
 

jnrhome

New Member
Csimmer:
The problem with that configuration is that there is no absorption at bulk voltage. As soon as you hit the max voltage, the BMS will disconnect and the voltage will drop back to something less than the bulk voltage. As soon as the voltage drops, the BMS restores charge and it will run right back up to max and the BMS will shut off again. The BMS protection circuit isn't meant to be triggered many times daily and will wear out long before the controller would. Most BMS's have less than great components and stressing them isn't a good idea. They are meant to stop a dangerous overcharge, not regulate your charge.

Bulk should be just below max and float should be just below that. If configured properly, the BMS will never (rarely) intervene.

I have my controller set to bulk-53.9v and float-53.8v with a bulk reconnect voltage at 53.7v My target maintain voltage is 53.6v and I have about .2v difference between controller measured voltage and battery measured voltage so 54.8v maintains my battery at ~53.6v. This causes my battery to be charged at bulk until just above my target voltage, then drop back slightly to float. It will stay in float unless a few clouds go by or until the end of the effective charge day. If clouds go by and the voltage drops far enough, bulk kicks back in and brings it up to just above float, then settles back in to float. At the end of the day, bulk kicks in and charges it at max until sundown, squeezing the last energy of the day in at bulk.
 

Csimmer

New Member
Csimmer:
The problem with that configuration is that there is no absorption at bulk voltage. As soon as you hit the max voltage, the BMS will disconnect and the voltage will drop back to something less than the bulk voltage. As soon as the voltage drops, the BMS restores charge and it will run right back up to max and the BMS will shut off again. The BMS protection circuit isn't meant to be triggered many times daily and will wear out long before the controller would. Most BMS's have less than great components and stressing them isn't a good idea. They are meant to stop a dangerous overcharge, not regulate your charge.

Bulk should be just below max and float should be just below that. If configured properly, the BMS will never (rarely) intervene.

I have my controller set to bulk-53.9v and float-53.8v with a bulk reconnect voltage at 53.7v My target maintain voltage is 53.6v and I have about .2v difference between controller measured voltage and battery measured voltage so 54.8v maintains my battery at ~53.6v. This causes my battery to be charged at bulk until just above my target voltage, then drop back slightly to float. It will stay in float unless a few clouds go by or until the end of the effective charge day. If clouds go by and the voltage drops far enough, bulk kicks back in and brings it up to just above float, then settles back in to float. At the end of the day, bulk kicks in and charges it at max until sundown, squeezing the last energy of the day in at bulk.
I agree with most of what you said. I have changed my setup since. I found an industrial contactor which I control with the relay in my Victron BMV700. The BMV700 relay can be triggered by battery voltage or STATE OF CHARGE (how cool is that?) it even has hysteresis settings. So I just set my MPP Solar to 56v bulk, 900min absorption and let the relay/contactor take care of the rest.
 

jnrhome

New Member
I'm still tweaking my settings. Currently reducing my boost voltage from 54v to 53.7v, and increasing the boost duration from 10 minutes to 30 to add more CC before floating without running the voltage up so high. Can't go too far or I won't get back to float during a cloudy period. I've been looking at relays and contactors as well. Hoping to find a simple, inexpensive one that I can trigger from my EpEver low voltage relay circuit to turn on an AC to DC power supply, and a second one to disconnect non-critical load.
 

rickst29

Solar Enthusiast
In my 'el-cheapo' EpEVER BN-series controller, running a 4s "12v" setup, the "boost mode" timer apparently will run for the full time specified. But after that, if the Battery Pack has reached the "Float Voltage" level, it will switch down to "Float Mode" and stay there until "Boost" is re-started by some load pulling the battery below "Boost Reconnect Voltage". Although the Monitor is capable of displaying the charging current, the operating microcode DOESN'T use that value to terminate "Boost Mode".

With LFP, balancing all the cells has occurred when "Boost Mode" current has reached zero. For LFP, there should (IMO), be at least one more parameter in EpEVER's programming: "Minimum Bulk Charging Current". It ain't there.

The battery pack is between a rock and a hard place. With the "Boost Mode" Voltage set at 14.5V for my battery pack, a very long "Boost Duration" time will assure that the cells all become balanced at the peak cell Voltage allowed by BMS (or nearly that much, given sufficient time). But after that has occurred, and current has reached zero, the EpEVER continues to pound the battery and BMS with the full 14.5V until the "Boost Duration" time has been reached.

OTOH, If I set the "Boost Duration" time too short, the battery pack will not be charged very far - EpEVER will see a battery Voltage of 13.8V (my Float Value) and drop to "Float stage", which slowly push only 3.45 Volts onto each cell.
- - - -
Anyway, my own current "compromise" values are: "Boost Voltage" = 14.5V, "Boost Duration" = 90 minutes (way longer than yours), "Float Voltage" = 13.8V, and "Boost Return Voltage" = 13.6V. My BMS seems to be OK at handling the excess Boost Duration, but my batteries spend most of the day at 100% SOC - which is not optimal for LFP batteries. Thank goodness, that problem only happens during actual trips.

[ In storage, my folding RV ("Trailmanor") is stored in the garage and not exposed to sun, I recharge to fill in for "phantom loads" only a few times per year. Those "phantom loads are tiny - just the full time heat controller Relay and a Coulomb Counter Battery Monitor. Everything else gets disabled by disconnecting the 12v distribution panel during storage. ]
 

GSXR1000

Solar Addict
i would say it is charging 28v then not charging until it reaches float then maintains that voltage.

i set my bulk at 13.80 and float at 13.73 so that it can stop charging but not discharge much before it tops it off again.
 

jnrhome

New Member
i've found that by setting the boost, float boost recon voltages close together, the controller hands off the CC function from a timed boost control to float mode. I adjusted my boost voltage to 53.9v, float to 53.85 and boost recon to 53.8, and my boost duration back to 10 minutes as the 30 minute timeframe only made it more difficult to get back into float after a passing cloud or surge in load. At 10 minute boost duration, float kicks in, but doesn't drop to load plus a few watts. Instead, the float mode current starts at the boost current level, slowly tapering downward until it matches the load draw plus a few watts. This gives me a graphed status of absorption level. Between initiation of float mode and wattage meeting the load amount, I can see just how saturated the cells are. My float stage is not very sensitive to over or under voltage and can surge above boost or recede to a full volt below the boost recon level and not drop back into boost. Variations in voltage during float simply increase or decrease the total wattage sent to the batteries, without shifting gears. The closer to full saturation I am, the more variation in voltage and SOC float will allow. Not sure if it's the right way to go about this, but i'm finding that the voltage is right at target near the end of the day, with boost kicking back in just before sunset. I've purchased a dc triggered ac SSR to try and add so if the voltage goes below 48, the SSR kicks in and delivers AC-DC power, adding some wattage to offset load, mostly right before sunrise as the battery approaches the 48v cliff. I can't get it to work tho as the Epever utility relay doesn't output the 30v/5a as the documentation states. Trying to work with Epever tech support on that now.
 
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