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--- electrical:12v:alternator_details [2025/06/18 15:27]
frater_secessus [heat] load dumps
+++ electrical:12v:alternator_details [2025/06/18 20:13] (current)
frater_secessus [load dumps]
@@ Line 225: / Line 225: @@
 ==== load dumps ====
+The alternator's voltage regulator responds to demand, increasing and decreasing field strength (and therefore output) as required to hold a given voltage setpoint.
+  * when **loads are added** chassis voltage will drop briefly while the regulator responds.
+  * when **loads are removed** chassis voltage will spike briefly while the regulator responds.
+  * if **large loads disappear suddenly** and nothing is available to absorb the spike that excess voltage can damage the alternator and/or other electrics.
+In typical installs there //are// places for the power in the alternator to go, chiefly the lead starter battery:
+> [lead] batteries have about 1000F ( farad ) [capacitance] per 150Ah of capacity so they act as very substantial capacitors.  Hence they have enormous ability to absorb spike energy.((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-7.html#post3533042)),((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-8.html#post3533143))
+The real problem is secondary alternators dedicated to LFP charging;  if the BMS disconnects charging there is no starter battery on that separate circuit to absorb the energy.
+For a typical install the  question is:  how much charging current are we talking about, and does the alternator already handle that level of sudden disconnect under normal operation? Secessus provides this example:
+> my van's radiator cooling fans are rated at 65A((https://www.promasterforum.com/threads/180-amp-alternator-with-amp-clamp-on-output.99118/post-792681)) and turn on/off frequently via relay.((see below))  My Li charging current is less than that.
+In addition, the speed of the disconnect plays a part:
+> there is a massive difference in the voltage spike from a 5 uS relay disconnect and a 10mS mosfet disconnect [as found in the BMS]((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-7.html#post3532864))
 When an alternator is outputting Big Power and the demand for that power vanishes the alternator (and other chassis electricals) can be damaged:
@@ Line 230: / Line 252: @@
 > When the rotor current is stoked up to create a large rotor magnetic field for high alternator output and there is a sudden drop off in alternator load, the stored rotor energy takes some time to dissipate. This causes a voltage spike in output of the alternator if there is suddenly little to no load on alternator... the voltage spike can be up to several hundred volts when loading suddenly drops off. This can damage alternator diodes by exceeding their maximum voltage breakdown rating as well as any electronic equipment powered on the alternator output DC bus.((https://diysolarforum.com/threads/alternator-dc-dc-charger-compared-to-buc-converter-whats-the-difference.103819/post-1448377))
-Normally the lead-acid starter battery absorbs enough of the spike that the load dump is a non-issue;  this is what happens when we turn off the headlights, the radiator cooling fan[s] cycle off, etc.
+=== approaches ===
-The problem is most critical when secondary alternators are dedicated to charging LFP.  If the BMS shuts down charging the alternator there is nothing to absorb the spike.  High-end systems use CANBUS or other networking whereion the BMS informs the alternator's regulator prior to actual shutdown.  The alt shuts down before the BMS so there are no spikes.
+  * use the lead starter batt in the circuit to absorb the spike
+  * charge the bank in a manner unlikely to cause BMS disconnect in the first place
+  * for LFP-dedicated alternator setups
+    * consider a system that uses CANBUS or other networking protocol to inform the alternator's regulator prior to actual BMS shutdown.  The alt shuts down before the BMS so there are no spikes.
+    * install a possibly-sacrificial protection device.  They are typically mounted between the alernator's B+ and B- terminals and short to ground when voltage spikes above a certain level.
+      * The  [[https://amzn.to/45ofEh4|Balmar APM-12]]   apparently uses "A TVS (avalanche diode), Littlefuse brand, unipolar, 15kW peak pulse capability, with a stand-off voltage around 20 or 22 Volts"((https://www.cruisersforum.com/forums/f14/balmar-alternator-protection-module-apm-260911-post3607461.html#post3607461))
+      * the [[https://amzn.to/4ectIwD|Balmar TSP-12 spike protector]] may be the equivalent of the APM-12 without the LED and audio indicators.
+      * the Zap-Stop apparently uses a Motorola #MR2535L TVS diode((the original post called it a Zener but [[https://www.alldatasheet.com/datasheet-pdf/view/5199/MOTOROLA/MR2535L.html|the datasheet for the MR2535L]] says it's TVS)) to short the alternator output((https://www.cruisersforum.com/forums/f14/zap-stop-fact-or-fiction-29442.html#post315900))
+      * Sterling APD12 internals not documented
+      * mikecol recommends a "IN6277A [[https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode|TVS diode]] in series with 5A ATC fuse. The fuse characteristics with the diode clamp makes the full circuit. I don't have data to show it works well, but worth the cheap cost for me. Fuse and diode might not survive after hit, so good to be able to check condition (I have fuse end to alt+ so can measure with voltmeter [easily]"((https://sprinter-source.com/forums/index.php?threads/85448/post-918529))
 ===== common modifications =====
@@ Line 381: / Line 411: @@
 {As far as I can tell this was a workaround from when many isolators were diode-based so the alternator could not "see" aux battery voltage on the combined circuit((https://www.foreforums.com/index.php?PHPSESSID=bs0n0psbr6rrvd240rv4ijvg1p&topic=33250.msg300875#msg300875)) -- secessus}  See [[https://www.dixie-electric.com/tsb/TSB_704.pdf|this TSB]]. (PDF)
-===== sudden load disconnect =====
-The alternator's voltage regulator responds to demand, increasing and descreasing field strength (and therefore output) as required to hold a given voltage setpoint.
-  * when **large loads are suddenly added** chassis voltage will drop briefly while the regulator responds.
-  * when **large loads are suddenly removed** chassis voltage will spike briefly while the regulator responds.  If the chassis cannot absorb this spike the excess power can be dissipated within the alternator itself and cause damage
-**In theory** if the BMS disconnects charging the resulting spike could damage the alternator.  Team Karst explains it colloquially:
->> Alternator is chugging along, delivering 50 Amps. Suddenly, the output is disconnected. Since the regulator has a time constant, plus the field current can’t collapse instantly, plus stator windings, being coils, therefore inductive, they hold energy, there becomes a high positive voltage at the B+ port to keep the current flowing for a time. This transient energy may manifest itself in switch arcs, and other undesirable voltage excursions.((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-6.html#post3532355))
-This is called a //load dump//; the load is dumped (demand reduced to 0A) and there is nowhere for power in the alternator to go.  As we will see, in normal installs there //are// places for the power in the alternator to go.
-**In practice**:
-  * [a wise user will not be bouncing off the BMS in the first place -- Secessus]
-  * the vast majority of lithium banks charged by alternator do so in combination with the starter battery.  The lead battery will help absorb spikes from the alternator. "Batteries have about 1000F ( farad ) [capacitance] per 150Ah of capacity so they act as very substantial capacitors.  Hence they have enormous ability to absorb spike energy."((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-7.html#post3533042)),((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-8.html#post3533143))
-  * modern alternators have avalanche diodes and fast-acting regulators that can adjust within 100ms.((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292.html#post3527179))
-  * if BMS disconnect  caused alternator damage [[electrical:12v:alternator#lithium-specific_vsr|the Li-Bim]] recommended by Battle Born would quickly eat alts.  It disconnects the bank from alternator every 35 minutes.((15min on, 20mins off))
-The question for the user is:  how much charging current are we talking about, and does the alternator already handle that level of sudden disconnect under normal operation? Secessus provides this example:
-> my van's radiator cooling fans are rated at 65A((https://www.promasterforum.com/threads/180-amp-alternator-with-amp-clamp-on-output.99118/post-792681)) and turn on/off frequently.  My Li charging current is less than that.
-In addition, the speed of the disconnect plays a part:
-> there is a massive difference in the voltage spike from a 5 uS relay disconnect and a 10mS mosfet disconnect((https://www.cruisersforum.com/forums/f166/myth-of-alternator-damage-cause-by-bms-disconnect-258292-7.html#post3532864))
-==== exception:  dedicated secondary alternators ====
-Secondary alternators dedicated to Li charging will not have a starter battery or other loads inline to absorb spikes.  In this case the owner might:
-  * use a CANBUS or similar setup where the BMS can notify the alternator regulator of imminent shutdown ahead of time to allow orderly reduction of power; or
-  * use a protection device, like [[https://www.sterling-power-usa.com
-/SterlingPower12voltalternatorprotectiondevice.aspx|the Sterling Alternator Protection Device]]; ([[https://www.sterling-power-usa.com/library/Sterling%20Power%20Alternator%20Protection%20Device.pdf|manual]]), [[https://balmar.net/product/apm-12/|Balmar APM]], etc.
-  * some maintain a small lead battery in parallel with the Li bank
-Note that the Sterling device uses "a small resistive load" (milliAmps) to control the spike.

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